Electrolyte solution, electrochemical device, lithium ion secondary battery and module

ABSTRACT

An electrolyte solution containing a solvent. The solvent contains a compound (1) represented by the following formula (1), wherein R a , R b , R c , and R d  are the same as or different from each other, and are each —H, —F, —CH 3 , or —CF 3 ; at least one of R a , R b , R c , or R d  is —F or —CF 3 ; and at least one of R a , R b , R c , or R d  is —CH 3 , and a compound (2) represented by the following formula (2), wherein R e  is a C1-C5 linear or branched alkyl or alkoxy group optionally containing an ether bond; R f  is a C1-C5 linear or branched alkyl group optionally containing an ether bond; and at least one of R e  or R f  contains a fluorine atom. Also disclosed is an electrochemical device including the electrolyte solution, a lithium-ion secondary battery including the electrolyte solution and a module including the electrochemical device.

TECHNICAL FIELD

The invention relates to electrolyte solutions, electrochemical devices,lithium-ion secondary batteries, and modules.

BACKGROUND ART

Current electric appliances demonstrate a tendency to have a reducedweight and a smaller size, which leads to development of electrochemicaldevices such as lithium-ion secondary batteries having a high energydensity. Further, electrochemical devices such as lithium-ion secondarybatteries are desired to have improved characteristics as they areapplied to more various fields. Improvement in battery characteristicswill become more and more important particularly when lithium-ionsecondary batteries are put in use for automobiles.

For example, Patent Literature 1 discloses an electrolyte solutioncontaining a solvent and an electrolyte salt, the solvent containing acyclic carbonate ester represented by the following formula:

(wherein R1 to R4 are each a hydrogen group, a fluorine group, an alkylgroup, or a fluorinated alkyl group, they are the same as or differentfrom each other, and at least one of them is a fluorine group or afluorinated alkyl group) and an acyclic carbonate ester represented bythe following formula:

(wherein R5 and R6 are each a hydrogen group, a fluorine group, an alkylgroup, or a fluorinated alkyl group, they are the same as or differentfrom each other, and at least one of them is a fluorine group or afluorinated alkyl group).

Patent Literature 2 discloses a nonaqueous electrolyte solutioncontaining: a solvent (I) for dissolving an electrolyte salt thatcontains a fluorine-containing ester solvent (A) represented by thefollowing formula:

R¹CFXCOOR²

(wherein R¹ is a hydrogen atom, a fluorine atom, or a C1-C3 alkyl groupin which a hydrogen atom is optionally replaced by a fluorine atom; X isa hydrogen atom or a fluorine atom; when R¹ is a fluorine atom or aperfluoroalkyl group, X is a hydrogen atom; and R² is a C1-C4 alkylgroup) and a fluorine-containing solvent (B) other than thefluorine-containing ester solvent (A); and an electrolyte salt (II).

Patent Literature 3 discloses an electrolyte solution containing asolvent and an electrolyte salt, the solvent containing afluorine-containing compound represented by any of the followingformulae:

Rf¹OCOOR

wherein Rf¹ is a C1-C4 fluorine-containing alkyl group; and R is a C1-C4non-fluorinated alkyl group; and

Rf²OCOORf³

wherein Rf² and Rf³ are the same as or different from each other, andare each a C1-C4 fluorine-containing alkyl group.

Patent Literature 4 discloses a non-aqueous electrolyte solutioncontaining: [A] a non-aqueous solvent that contains a fluorinatecarbonate, a cyclic carbonate, and an acyclic carbonate, with (i) thecyclic carbonate being contained in an amount of 2 to 63 mol %, (ii) theacyclic carbonate being contained in an amount of 2 to 63 mol %, and(iii) the fluorinated carbonate being contained in an amount of 60 to 96mol % (the sum of (i) to (iii) being smaller than 100 mol %), thefluorinated carbonate being represented by the following formula [1]:

(wherein R¹ and R² are the same as or different from each other; one ofthem is a C1-C4 hydrocarbon group in which one or more hydrogen atomsbeing replaced by fluorine atoms, and the other is a C1-C4 hydrocarbongroup or a 1-4 hydrocarbon group in which one or more hydrogen atomsbeing replaced by fluorine atoms, with such hydrocarbon groups includinggroups containing a hetero atom such as oxygen or nitrogen); and [B] anelectrolyte.

CITATION LIST Patent Literature Patent Literature 1: JP 2008-123714 APatent Literature 2: JP 2008-257988 A Patent Literature 3: WO2013/157503 Patent Literature 4: JP 4392726 B SUMMARY OF INVENTIONTechnical Problem

Conventional electrolyte solutions containing a fluorine-based solventgive excellent battery cycle characteristics, but tend to give a higherresistance than electrolyte solutions containing non-fluorine-basedsolvent. Those capable of reducing generation of gas during cycles areawaited.

The invention is made in view of the above state of the art, and aims toprovide an electrolyte solution that can give a low resistance to anelectrochemical device while maintaining the cycle characteristics ofthe electrochemical device and can reduce generation of gas duringcycles although the electrolyte solution contains a fluorine-basedsolvent, and an electrochemical device containing the electrolytesolution.

Solution to Problem

The inventors found that an electrolyte solution containing afluorinated cyclic carbonate having a specific structure and afluorinated acyclic ester or carbonate having a specific structure cangive a low resistance to an electrochemical device while maintaining thecycle characteristics of the electrochemical device and can reducegeneration of gas during cycles although the electrolyte solutioncontains a fluorine-based solvent.

Specifically, the invention relates to an electrolyte solutioncontaining a solvent, the solvent containing: a compound (1) representedby the following formula (1):

(wherein R^(a), R^(b), R^(c), and R^(d) are the same as or differentfrom each other, and are each —H, —F, —CH₃, or —CF₃; at least one ofR^(a), R^(b), R^(c), or R^(d) is F or —CF₃; and at least one of R^(a),R^(b), R^(c), or R^(d) is —CH₃); and a compound (2) represented by thefollowing formula (2):

(wherein R^(e) is a C1-C5 linear or branched alkyl or alkoxy groupoptionally containing an ether bond; R^(f) is a C1-C5 linear or branchedalkyl group optionally containing an ether bond; and at least one ofR^(e) or R^(f) contains a fluorine atom).

In the electrolyte solution of the invention, the compound (1) ispreferably contained in an amount of 0.001 to 99.999% by volume relativeto the solvent and the compound (2) is preferably contained in an amountof 0.001 to 99.999% by volume relative to the solvent.

The compound (1) preferably includes at least one selected from thegroup consisting of compounds represented by any of the followingformulae (1a), (1b), (1c), (1d), (1e), and (1f).

The electrolyte solution of the invention preferably further contains anelectrolyte salt.

The invention also relates to an electrochemical device including theelectrolyte solution. The invention also relates to a lithium-ionsecondary battery including the electrolyte solution.

The invention also relates to a module including the electrochemicaldevice or the lithium-ion secondary battery.

Advantageous Effects of Invention

The electrolyte solution of the invention has any of the abovestructures, and thus can give a low resistance to an electrochemicaldevice while maintaining the cycle characteristics of theelectrochemical device and can reduce generation of gas during cyclesalthough the electrolyte solution contains a fluorine-based solvent. Anelectrochemical device containing the electrolyte solution has excellentcycle characteristics and a low resistance, and less generates gasduring cycles.

DESCRIPTION OF EMBODIMENTS

The invention will be specifically described hereinbelow.

The electrolyte solution of the invention contains a solvent.

The solvent contains a compound (1) represented by the following formula(1):

wherein R^(a), R^(b), R^(c), and R^(d) are the same as or different fromeach other, and are each —H, —F, —CH₃, or —CF₃; at least one of R^(a),R^(b), R^(c), or R^(d) is —F or —CF₃; and at least one of R^(a), R^(b),R^(c), or R^(d) is —CH₃.

The compound (1) represented by the formula (1) may include one compoundor may include a combination of two or more compounds.

In order to give more improved cycle characteristics and a much lowerresistance and to further reduce generation of gas, the compound (1)preferably includes at least one selected from the group consisting ofcompounds represented by any of the following formulae (1a), (1b), (1c),(1d), (1e), and (1f).

In order to give a much lower resistance to an electrochemical device,particularly preferred among the compounds represented by any of theformulae (1a) to (1f) is at least one selected from the group consistingof the compound represented by the formula (1a) and the compoundrepresented by the formula (1c).

The compound represented by the formula (1a) among the compounds (1) canbe synthesized by charging hydroxyacetone, pyridine, and methylenechloride into an autoclave, introducing carbonyl fluoride thereinto, andheating the components.

The compound represented by the formula (1b) can be synthesized byadding tetrahydrofuran to trifluoromethyl ethylene carbonate, stirringthe components, adding potassium t-butoxide and iodomethane, andstirring the components.

The compounds (1) other than the compound represented by the formula(1a) and the compound represented by the formula (1b) each can besynthesized using an appropriate starting material instead ofhydroxyacetone or trifluoromethyl ethylene carbonate. A known synthesismethod may be used.

The solvent contains a compound (2) represented by the following formula(2):

wherein R^(e) is a C1-C5 linear or branched alkyl or alkoxy groupoptionally containing an ether bond; R^(f) is a C1-C5 linear or branchedalkyl group optionally containing an ether bond; and at least one ofR^(e) or R^(f) contains a fluorine atom.

The compound (2) preferably has a fluorine content of 10 to 70% by mass.The compound (2) having a fluorine content within the above range canlead to improved high-temperature storage characteristics and cyclecharacteristics of an electrochemical device. The lower limit of thefluorine content is more preferably 15% by mass, still more preferably25% by mass, particularly preferably 33% by mass. The upper limit of thefluorine content is more preferably 60% by mass, still more preferably45% by mass.

The fluorine content is a value calculated based on the structuralformula of the compound (2) by the following formula:

{(Number of fluorine atoms×19)/(molecular weight ofcompound(2))}×100(%).

When R^(e) is an alkoxy group, examples of R^(e) includefluorine-containing alkoxy groups such as CF₃O—, CF₃CH₂O—, H₂CFCH₂O—,CF₂HCH₂O—, HCF₂CF₂CH₂O—, CF₃CF₂CH₂O—, (CF₃)₂CHO—, H(CF₂CF₂)₂CH₂O—,CF₃CF₂O—, and FCH₂O—, fluorine-containing alkoxy groups containing anether bond such as C₂F₅OCF(CF₃)CH₂O— and CF₃OCF(CF₃)CH₂O—, andfluorine-free alkoxy groups such as CH₃O—, C₂H₅O—, C₃H₇O—, and C₄H₅O—.

When R^(e) is an alkoxy group, examples of R^(f) includefluorine-containing alkyl groups such as CF₃—, CH₂F—, CF₂H—, CF₃CH₂—,H₂CFCH₂—, HCF₂CH₂—, HCF₂CF₂CH₂—, CF₃CF₂CH₂—, (CF₃)₂CH—, H(CF₂CF₂)₂CH₂—,and CF₃CF₂—, fluorine-containing alkyl groups containing an ether bondsuch as CF₃OCH₂CH₂—, CF₃CH₂OCH₂—, C₂F₅OCF(CF₃)CH₂—, and CF₃OCF(CF₃)CH₂—,and fluorine-free alkyl groups such as CH₃—, C₂H₅—, C₃H₇—, and C₄H₅—.

Any combination of the above groups is selected so as to give a fluorinecontent within the above range.

When R^(e) is an alkoxy group, specific examples of the compound (2)include H₂CFOCOOCH₃, HCF₂OCOOCH₃, CF₃OCOOCH₃, H₂CFCH₂OCOOCH₃,HCF₂CH₂OCOOCH₃, CF₃CH₂OCOOCH₃, (CF₃CH₂O)₂CO, (HCF₂CF₂CH₂O)₂CO,(CF₃CF₂CH₂O)₂CO, ((CF₃)₂CHO)₂CO, (H(CF₂CF₂)₂CH₂O)₂CO, CH₃OCOOCH₂CF₂CF₃,CH₃OCOOCH₂CF₂CF₂H, C₂H₅OCOOCH₂CF₂CF₂H, C₂H₅OCOOCH₂CF₃,CF₃CF₂CH₂OCOOCH₂CF₂CF₂H, HCF₂CF₂CH₂OCOOC₃H₇, and (CF₃)₂CHOCOOCH₃.

Preferred among these is at least one selected from the group consistingof H₂CFOCOOCH₃, (CF₃CH₂O)₂CO, (HCF₂CF₂CH₂O)₂CO, (CF₃CF₂CH₂O)₂CO,((CF₃)₂CHO)₂CO, (H(CF₂CF₂)₂CH₂O)₂CO, CH₃OCOOCH₂CF₂CF₃,CH₃OCOOCH₂CF₂CF₂H, C₂H₅OCOOCH₂CF₂CF₂H, CH₃OCOOCH₂CF₃, C₂H₅OCOOCH₂CF₃,CF₃CF₂CH₂OCOOCH₂CF₂CF₂H, HCF₂CF₂CH₂OCOOC₃H₇, (CF₃)₂CHOCOOCH₃, andCF₃CH₂OCOOCH₃.

More preferred is at least one selected from the group consisting ofH₂CFOCOOCH₃, CF₃CH₂OCOOCH₃, and (CF₃CH₂O)₂CO.

When R^(e) is an alkyl group, examples of R^(e) include H₂CF—, HCF₂—,CF₃—, CF₃CF₂—, HCF₂CF₂—, CH₃CF₂—, HCF₂CH₂—, H₂CFCH₂—, and CF₃CH₂—. Inorder to give a low resistance, good viscosity, and oxidation resistanceto an electrochemical device, particularly preferred are HCF₂—, CF₃—,CH₃CF₂—, CF₃CH₂—, and HCF₂CH₂—.

When R^(e) is an alkyl group, examples of R^(f) include CH₃—, C₂H₅—,CF₃—, CF₃CF₂—, (CF₃)₂CH—, CF₃CH₂—, CF₃CH₂CH₂—, CF₃CFHCF₂CH₂—, C₂F₅CH₂—,CF₂HCF₂CH₂—, C₂F₅CH₂CH₂—, CF₃CF₂CH₂—, and CF₃CF₂CF₂CH₂—. In order togive a low resistance to an electrochemical device and to achieve goodmiscibility with other solvents, particularly preferred are CH₃—, C₂H₅—,CF₃CH₂—, and CF₃CH₂CH₂—.

When R^(e) is an alkyl group, specific examples of the compound (2)include one or two or more of CF₃CH₂C(═O)OCH₃, HCF₂C(═O)OCH₃,CF₃C(═O)OCH₂CH₂CF₃, HCF₂CH₂C(═O)OCH₃, CF₃C(═O)OCH₂C2F₅, CF₃C(═O)OCH₂CF₂CF₂H, CF₃C(═O)OCH₂CF₃, and CF₃C(═O)OCH(CF₃)₂. In order to give alow resistance and rate characteristics to an electrochemical device andto achieve good miscibility with other solvents, particularly preferredare CF₃CH₂C(═O)OCH₃, HCF₂C(═O)OCH₃, CF₃C(═O) OCH₂C₂F₅, CF₃C(═O)OCH₂CF₂CF₂H, CF₃C(═O) OCH₂CF₃, CF₃C(═O)OCH(CF₃)₂, and HCF₂CH₂C(═O)OCH₃.

The compound (2) represented by the formula (2) may include one compoundor two or more compounds in combination.

The compound (2) can be synthesized by a conventionally known method.

The compound (1) is preferably contained in an amount of 0.001 to99.999% by volume, more preferably 0.01 to 99.99% by volume, relative tothe solvent. In order to give a much lower resistance to anelectrochemical device, the amount thereof is still more preferably 0.1to 70% by volume, further more preferably 1 to 60% by volume, stillfurther more preferably 15 to 55% by volume, particularly preferably 20to 50% by volume, relative to the solvent.

When two or more compounds are used in combination as the compound (1),the sum of the two or more compounds falls within the above range.

The compound (2) is preferably contained in an amount of 0.001 to99.999% by volume, more preferably 0.01 to 99.99% by volume, relative tothe solvent. In order to give a much lower resistance to anelectrochemical device, the amount thereof is still more preferably 30to 99.9% by volume, further more preferably 40 to 99% by volume, stillfurther more preferably 45 to 85% by volume, particularly preferably 50to 70% by volume, relative to the solvent.

When two or more compounds are used in combination as the compound (2),the sum of the two or more compounds falls within the above range.

The sum of the amounts of the compound (1) and the compound (2) ispreferably 10 to 100% by volume relative to the solvent. In order togive a much lower resistance to an electrochemical device, the sum ismore preferably 40 to 99% by volume, still more preferably 60 to 95% byvolume, relative to the solvent.

The solvent in the invention may further contain at least one selectedfrom the group consisting of a non-fluorinated saturated cycliccarbonate and a non-fluorinated acyclic carbonate.

Examples of the non-fluorinated saturated cyclic carbonate includenon-fluorinated saturated cyclic carbonates containing a C2-C4 alkylenegroup.

In order to give a high permittivity and good viscosity, thenon-fluorinated saturated cyclic carbonate preferably includes at leastone selected from the group consisting of ethylene carbonate, propylenecarbonate, and butylene carbonate.

The non-fluorinated saturated cyclic carbonates may be used alone or inany combination of two or more at any ratio.

Examples of the non-fluorinated acyclic carbonate includehydrocarbon-based acyclic carbonates such as CH₃OCOOCH₃ (dimethylcarbonate, DMC), CH₃CH₂OCOOCH₂CH₃ (diethyl carbonate, DEC),CH₃CH₂OCOOCH₃ (ethyl methyl carbonate, EMC), CH₃OCOOCH₂CH₂CH₃ (methylpropyl carbonate), methyl butyl carbonate, ethyl propyl carbonate, andethyl butyl carbonate. Preferred is at least one selected from the groupconsisting of ethyl methyl carbonate, diethyl carbonate, and dimethylcarbonate.

The non-fluorinated acyclic carbonates may be used alone or in anycombination of two or more at any ratio.

The non-fluorinated saturated cyclic carbonate is preferably in anamount of 0 to 90% by volume, more preferably 1 to 60% by volume, stillmore preferably 5 to 40% by volume, relative to the solvent.

The non-fluorinated acyclic carbonate is preferably in an amount of 0 to90% by volume, more preferably 1 to 60% by volume, still more preferably5 to 40% by volume, relative to the solvent.

The sum of the non-fluorinated saturated cyclic carbonate and thenon-fluorinated acyclic carbonate is preferably 0 to 90% by volume, morepreferably 1 to 60% by volume, still more preferably 5 to 40% by volume,relative to the solvent.

The solvent in the invention may further contain at least one selectedfrom the group consisting of a non-fluorinated saturated cyclic esterand a non-fluorinated acyclic ester.

Examples of the non-fluorinated saturated cyclic ester includenon-fluorinated saturated cyclic esters containing a C2-C4 alkylenegroup.

Specific examples of the non-fluorinated saturated cyclic esterscontaining a C2-C4 alkylene group include β-propiolactone,γ-butyrolactone, and δ-valerolactone. In order to improve the degree ofdissociation of lithium ions and to improve the load characteristics,particularly preferred are γ-butyrolactone and δ-valerolactone.

The non-fluorinated saturated cyclic esters may be used alone or in anycombination of two or more at any ratio.

Examples of the non-fluorinated acyclic ester include methyl acetate,ethyl acetate, butyl acetate, methyl propionate, ethyl propionate,propyl propionate, and butyl propionate.

Preferred among these are ethyl propionate and propyl propionate,particularly preferred is ethyl propionate.

The non-fluorinated acyclic esters may be used alone or in anycombination of two or more at any ratio.

The non-fluorinated saturated cyclic ester is preferably in an amount of0 to 90% by volume, more preferably 0.001 to 90% by volume, still morepreferably 1 to 60% by volume, particularly preferably 5 to 40% byvolume, relative to the solvent.

The non-fluorinated acyclic ester is preferably in an amount of 0 to 90%by volume, more preferably 0.001 to 90% by volume, still more preferably1 to 60% by volume, particularly preferably 5 to 40% by volume, relativeto the solvent.

The sum of the non-fluorinated saturated cyclic ester and thenon-fluorinated acyclic ester is preferably 0 to 90% by volume, morepreferably 1 to 60% by volume, still more preferably 5 to 40% by volume,relative to the solvent.

The solvent in the invention may further contain a fluorinated saturatedcyclic carbonate other than the above compound (1).

The fluorinated saturated cyclic carbonate other than the above compound(1) is a saturated cyclic carbonate containing a fluorine atom. Specificexamples thereof include a fluorinated saturated cyclic carbonate (A)(other than the compound (1)) represented by the following formula (A):

(wherein X¹ to X⁴ are the same as or different from each other, and areeach —H, —CH₃, —C₂H₅, —F, a fluorinated alkyl group optionallycontaining an ether bond, or a fluorinated alkoxy group optionallycontaining an ether bond; at least one of X¹ to X⁴ is —F, a fluorinatedalkyl group optionally containing an ether bond, or a fluorinated alkoxygroup optionally containing an ether bond). Examples of the fluorinatedalkyl group include —CF₃, —CF₂H, and —CH₂F.

The presence of the fluorinated saturated cyclic carbonate (A) in theelectrolyte solution of the invention when applied to a lithium-ionsecondary battery, for example, can lead to improved oxidationresistance of the electrolyte solution, providing stable, excellentcharge and discharge characteristics.

The term “ether bond” herein means a bond represented by —O—.

In the formula (A), in order to achieve good permittivity and oxidationresistance, one or two of X¹ to X⁴ is/are each preferably —F, afluorinated alkyl group optionally containing an ether bond, or afluorinated alkoxy group optionally containing an ether bond.

In the formula (A), in anticipation of a decrease in viscosity at lowtemperature, an increase in flash point, and improvement in solubilityof an electrolyte salt, X¹ to X⁴ are each preferably —H, —F, afluorinated alkyl group (a), a fluorinated alkyl group (b) containing anether bond, or a fluorinated alkoxy group (c).

The fluorinated alkyl group (a) is an alkyl group in which at least onehydrogen atom is replaced by a fluorine atom. The fluorinated alkylgroup (a) preferably has a carbon number of 1 to 20, more preferably 2to 17, still more preferably 2 to 7, particularly preferably 2 to 5.

Too large a carbon number may cause poor low-temperature characteristicsand low solubility of the electrolyte salt. Too small a carbon numbermay cause low solubility of the electrolyte salt, low dischargeefficiency, and increased viscosity, for example.

Examples of the fluorinated alkyl group (a) having a carbon number of 1include CFH₂—, CF₂H—, and CF₃—.

In order to achieve good solubility of the electrolyte salt, preferredexamples of the fluorinated alkyl group (a) having a carbon number of 2or greater include fluorinated alkyl groups represented by the followingformula (a-1):

R¹—R²—  (a-1)

wherein R¹ is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom; R² is a C1-C3 alkylene groupoptionally containing a fluorine atom; and at least one selected from R¹and R² contains a fluorine atom.

R¹ and R² each may further contain an atom other than carbon, hydrogen,and fluorine atoms.

R¹ is an alkyl group having a carbon number of 1 or greater andoptionally containing a fluorine atom. R¹ is preferably a C1-C16 linearor branched alkyl group. The carbon number of R¹ is more preferably 1 to6, still more preferably 1 to 3.

Specifically, for example, CH₃—, CH₃CH₂—, CH₃CH₂CH₂—, CH₃CH₂CH₂CH₂—, andgroups represented by the following formulae:

may be mentioned as linear or branched alkyl groups for R¹.

Examples of R¹ which is a linear alkyl group containing a fluorine atominclude CF₃—, CF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—,CF₃CH₂CF₂—, CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—,CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—,CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂—,HCF₂CH₂—, HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—,HCF₂CF₂CH₂CH₂—, HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—,HCF₂CH₂CF₂CH₂CH₂—, HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂—,FCH₂CH₂—, FCH₂CF₂—, FCH₂CF₂CH₂—, FCH₂CF₂CF₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—,CH₃CH₂CH₂—, CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—,CH₃CF₂CH₂CF₂CH₂—, CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CF₂CH₂CF₂CH₂—CH₃CF₂CH₂CF₂CH₂CH₂—, CH₃CF₂CH₂CF₂CH₂CH₂—, HCFClCF₂CH₂—,HCF₂CFCl CH₂—, HCF₂CFCl CF₂CFCl CH₂—, and HCFClCF₂CFCl CF₂CH₂—.

Examples of R¹ which is a branched alkyl group containing a fluorineatom include those represented by the following formulae.

The presence of a branch such as —CH₃ or —CF₃ may easily cause highviscosity. Thus, the number of such branches is more preferably small(one) or zero.

R² is a C1-C3 alkylene group optionally containing a fluorine atom. R²may be either linear or branched. Examples of a minimum structural unitconstituting such a linear or branched alkylene group are shown below.R² is constituted by one or combination of these units.

(i) Linear Minimum Structural Units

—CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—

(ii) Branched Minimum Structural Units

Preferred among these exemplified units are Cl-free structural unitsbecause such units may not be dehydrochlorinated by a base, and thus maybe more stable.

R² which is a linear group consists only of any of the above linearminimum structural units, and is preferably —CH₂—, —CH₂CH₂—, or CF₂—. Inorder to further improve the solubility of the electrolyte salt, —CH₂—or —CH₂CH₂— is more preferred.

R² which is a branched group includes at least one of the above branchedminimum structural units. A preferred example thereof is a grouprepresented by —(CX^(a)X^(b))—, wherein X^(a) is H, F, CH₃, or CF₃;X^(b) is CH₃ or CF₃; when X^(b) is CF₃, X^(a) is H or CH₃. Such a groupcan much further improve the solubility of the electrolyte salt.

For example, CF₃CF₂—, HCF₂CF₂—, H₂CFCF₂—, CH₃CF₂—, CF₃CF₂CF₂—,HCF₂CF₂CF₂—, H₂CFCF₂CF₂—, CH₃CF₂CF₂—, and those represented by thefollowing formulae:

may be mentioned as preferred examples of the fluorinated alkyl group(a).

The fluorinated alkyl group (b) containing an ether bond is an alkylgroup containing an ether bond in which at least one hydrogen atom isreplaced by a fluorine atom. The fluorinated alkyl group (b) containingan ether bond preferably has a carbon number of 2 to 17. Too large acarbon number may cause high viscosity of the fluorinated saturatedcyclic carbonate (A). This may also cause the presence of manyfluorine-containing groups, resulting in poor solubility of theelectrolyte salt due to reduction in permittivity, and poor miscibilitywith other solvents. Accordingly, the carbon number of the fluorinatedalkyl group (b) containing an ether bond is preferably 2 to 10, morepreferably 2 to 7.

The alkylene group which constitutes the ether moiety of the fluorinatedalkyl group (b) containing an ether bond is a linear or branchedalkylene group. Examples of a minimum structural unit constituting sucha linear or branched alkylene group are shown below.

(i) Linear Minimum Structural Units

—CH₂—, —CHF—, —CF₂—, —CHCl—, —CFCl—, —CCl₂—

(ii) Branched Minimum Structural Units

The alkylene group may be constituted by one of these minimum structuralunits, or may be constituted by multiple linear units (i), by multiplebranched units (ii), or by a combination of a linear unit (i) and abranched unit (ii). Preferred examples will be mentioned in detaillater.

Preferred among these exemplified units are Cl-free structural unitsbecause such units may not be dehydrochlorinated by a base, and thus maybe more stable.

A still more preferred example of the fluorinated alkyl group (b)containing an ether bond is a group represented by the following formula(b-1):

R³—(OR⁴)_(n1)—  (b-1)

wherein R³ is preferably a C1-C6 alkyl group optionally containing afluorine atom; R⁴ is preferably a C1-C4 alkylene group optionallycontaining a fluorine atom; n1 is an integer of 1 to 3; and at least oneselected from R³ and R⁴ contains a fluorine atom.

Examples of R³ and R⁴ include the following groups, and any appropriatecombination of these groups can provide the fluorinated alkyl group (b)containing an ether bond represented by the formula (b-1). Still, thegroups are not limited thereto.

(1) R³ is preferably an alkyl group represented by X^(c) ₃C—(R⁵)_(n2)—,wherein three X^(c)s are the same as or different from each other, andare each H or F; R⁵ is a C1-C5 alkylene group optionally containing afluorine atom; and n2 is 0 or 1.

When n2 is 0, R³ may be CH₃—, CF₃—, HCF₂—, or H₂CF—, for example.

When n2 is 1, specific examples of R³ which is a linear group includeCF₃CH₂—, CF₃CF₂—, CF₃CH₂CH₂—, CF₃CF₂CH₂—, CF₃CF₂CF₂—, CF₃CH₂CF₂—,CF₃CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CH₂CF₂CH₂—, CF₃CF₂CF₂CH₂—,CF₃CF₂CF₂CF₂—, CF₃CF₂CH₂CF₂—, CF₃CH₂CH₂CH₂CH₂—, CF₃CF₂CH₂CH₂CH₂—,CF₃CH₂CF₂CH₂CH₂—, CF₃CF₂CF₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂—, CF₃CF₂CH₂CF₂CH₂—,CF₃CF₂CH₂CH₂CH₂CH₂—, CF₃CF₂CF₂CF₂CH₂CH₂—, CF₃CF₂CH₂CF₂CH₂CH₂—, HCF₂CH₂—,HCF₂CF₂—, HCF₂CH₂CH₂—, HCF₂CF₂CH₂—, HCF₂CH₂CF₂—, HCF₂CF₂CH₂CH₂—,HCF₂CH₂CF₂CH₂—, HCF₂CF₂CF₂CF₂—, HCF₂CF₂CH₂CH₂CH₂—, HCF₂CH₂CF₂CH₂CH₂—,HCF₂CF₂CF₂CF₂CH₂—, HCF₂CF₂CF₂CF₂CH₂CH₂—, FCH₂CH₂—, FCH₂CF₂—,FCH₂CF₂CH₂—, CH₃CF₂—, CH₃CH₂—, CH₃CF₂CH₂—, CH₃CF₂CF₂—, CH₃CH₂CH₂—,CH₃CF₂CH₂CF₂—, CH₃CF₂CF₂CF₂—, CH₃CH₂CF₂CF₂—, CH₃CH₂CH₂CH₂—,CH₃CF₂CH₂CF₂CH₂—, CH₃CF₂CF₂CF₂CH₂—, CH₃CF₂CF₂CH₂CH₂—, CH₃CH₂CF₂CF₂CH₂—,CH₃CF₂CH₂CF₂CH₂CH₂—, and CH₃CH₂CF₂CF₂CH₂CH₂—.

When n2 is 1, those represented by the following formulae:

may be mentioned as examples of R³ which is a branched group.

A group having a branch such as —CH₃ or —CF₃ may easily cause highviscosity. Thus, R³ is more preferably a linear group.

(2) In —(OR⁴)_(n1)— of the formula (b-1), n1 is an integer of 1 to 3,preferably 1 or 2. When n1 is 2 or 3, R⁴s may be the same as ordifferent from each other.

Preferred specific examples of R⁴ include the following linear orbranched groups.

Examples of the linear groups include —CH₂—, —CHF—, —CF₂—, —CH₂CH₂—,—CF₂CH₂—, —CF₂CF₂—, —CH₂CF₂—, —CH₂CH₂CH₂—, —CH₂CH₂CF₂—, —CH₂CF₂CH₂—,—CH₂CF₂CF₂—, —CF₂CH₂CH₂—, —CF₂CF₂CH₂—, —CF₂CH₂CF₂—, and —CF₂CF₂CF₂—.

Those represented by the following formulae:

may be mentioned as examples of the branched groups.

The fluorinated alkoxy group (c) is an alkoxy group in which at leastone hydrogen atom is replaced by a fluorine atom. The fluorinated alkoxygroup (c) preferably has a carbon number of 1 to 17. The carbon numberis more preferably 1 to 6.

The fluorinated alkoxy group (c) is particularly preferably afluorinated alkoxy group represented by X^(d) ₃C—(R⁶)_(n3)—O—, whereinthree X^(d)s are the same as or different from each other, and are eachH or F; R⁶ is preferably a C1-C5 alkylene group optionally containing afluorine atom; n3 is 0 or 1; and any of the three X^(d)s contain afluorine atom.

Specific examples of the fluorinated alkoxy group (c) includefluorinated alkoxy groups in which an oxygen atom binds to an end of analkyl group mentioned as an example for R¹ in the formula (a-1).

The fluorinated alkyl group (a), the fluorinated alkyl group (b)containing an ether bond, and the fluorinated alkoxy group (c) in thefluorinated saturated cyclic carbonate (A) each preferably have afluorine content of 10% by mass or more. Too low a fluorine content maycause a failure in sufficiently achieving an effect of increasing theflash point. Thus, the fluorine content is more preferably 20% by massor more, still more preferably 30% by mass or more. The upper limitthereof is usually 85% by mass.

The fluorine content of each of the fluorinated alkyl group (a), thefluorinated alkyl group (b) containing an ether bond, and thefluorinated alkoxy group (c) is a value calculated based on thecorresponding structural formula by the following formula:

{(Number of fluorine atoms×19)/(formula weight of group)}×100(%).

The fluorine content in the whole fluorinated saturated cyclic carbonate(A) is preferably 5% by mass or more, more preferably 10% by mass ormore. The upper limit thereof is usually 76% by mass. In order toachieve good permittivity and oxidation resistance, the fluorine contentin the whole fluorinated saturated cyclic carbonate (A) is preferably 10to 70% by mass, more preferably 15 to 60% by mass.

The fluorine content in the fluorinated saturated cyclic carbonate (A)is a value calculated based on the structural formula of the fluorinatedsaturated cyclic carbonate (A) by the following formula:

{(Number of fluorine atoms×19)/(molecular weight of fluorinatedsaturated cyclic carbonate(A)}×100(%).

Specific examples of the fluorinated saturated cyclic carbonate (A)include the following.

In the formula (A), specific examples of the fluorinated saturatedcyclic carbonate (A) in which at least one of X¹ to X⁴ is —F includethose represented by the following formulae.

These compounds have a high withstand voltage and give good solubilityof the electrolyte salt.

Alternatively, those represented by the following formulae:

may also be used.

Those represented by the following formulae:

may be mentioned as specific examples of the fluorinated saturatedcyclic carbonate in which at least one of X¹ to X⁴ is a fluorinatedalkyl group (a) and the others thereof are —H.

Those represented by the following formulae:

may be mentioned as specific examples of the fluorinated saturatedcyclic carbonate in which at least one of X¹ to X⁴ is a fluorinatedalkyl group (b) containing an ether bond or a fluorinated alkoxy group(c) and the others thereof are —H.

In particular, the fluorinated saturated cyclic carbonate is preferablyany of the following compounds.

The fluorinated saturated cyclic carbonate (A) is preferablyfluoroethylene carbonate, difluoroethylene carbonate, or4-trifluoromethyl-ethylene carbonate.

The fluorinated saturated cyclic carbonate (A) is not limited to theabove specific examples. The above fluorinated saturated cycliccarbonates (A) may be used alone or in any combination of two or more atany ratio.

In the electrolyte solution of the invention, the fluorinated saturatedcyclic carbonate (A) is preferably in an amount of 0 to 50% by volume,more preferably 1 to 50% by volume, still more preferably 5 to 30% byvolume, particularly preferably 10 to 20% by volume, relative to thesolvent. The fluorinated saturated cyclic carbonate (A) in an amountwithin the above range can improve the oxidation resistance of theelectrolyte solution.

The solvent in the invention may further contain a fluorinated acycliccarbonate other than the compound (2). Examples of such a fluorinatedacyclic carbonate include (C₃F₇OCF(CF₃)CF₂OCF(CF₃)CH₂O)₂CO,(C₃F₇OCF(CF₃)CH₂O)₂CO, C₃F₇OCF(CF₃)CH₂OCOOC₃H₇, and(C₃F₇OCF(CF₃)CH₂O)₂CO.

The solvent in the invention may further contain a fluorinated acyclicester other than the compound (2).

In the solvent, preferably, the amount of the compound (1) is 0.5 to 65%by volume, the amount of the compound (2) is 35 to 85% by volume, andthe amount of the compound other than the compounds (1) and (2) is 0 to35% by volume.

The compound other than the compounds (1) and (2) preferably includes atleast one selected from the group consisting of the aforementionednon-fluorinated saturated cyclic carbonate, non-fluorinated acycliccarbonate, non-fluorinated acyclic ester, and fluorinated cycliccarbonate (A), more preferably include at least one selected from thegroup consisting of ethylene carbonate, propylene carbonate, ethylmethyl carbonate, dimethyl carbonate, ethyl propionate, fluoroethylenecarbonate, trifluoromethyl ethylene carbonate, and difluoroethylenecarbonate.

When the solvent consists only of the compound (1) and the compound (2),preferably, the amount of the compound (1) is 15 to 40% by volume andthe amount of the compound (2) is 60 to 85% by volume relative to thesolvent.

When the solvent contains a compound other than the compound (1) and thecompound (2), preferably, the amount of the compound (1) is 0.5 to 30%by volume, the amount of the compound (2) is 50 to 70% by volume, andthe amount of the compound other than the compounds (1) and (2) is 10 to35% by volume, relative to the solvent.

The solvent is preferably a non-aqueous solvent, and the electrolytesolution of the invention is preferably a non-aqueous electrolytesolution.

The electrolyte solution of the invention preferably further contains anelectrolyte salt. Examples of the electrolyte salt used include lithiumsalts, ammonium salts, and metal salts, as well as any of those to beused for an electrolyte solution such as liquid salts (ionic liquids),inorganic polymer salts, and organic polymer salts.

The electrolyte salt of the electrolyte solution for a lithium-ionsecondary battery is preferably a lithium salt.

Any lithium salt may be used. Specific examples thereof include thefollowing:

inorganic lithium salts such as LiPF₆, LiBF₄, LiClO₄, LiAlF₄, LiSbF₆,LiTaF₆, and LiWF₇;

lithium tungstates such as LiWOF₅;

lithium carboxylates such as HCO₂Li, CH₃CO₂Li, CH₂FCO₂Li, CHF₂CO₂Li,CF₃CO₂Li, CF₃CH₂CO₂Li, CF₃CF₂CO₂Li, CF₃CF₂CF₂CO₂Li, andCF₃CF₂CF₂CF₂CO₂Li;

lithium sulfonates such as FSO₃Li, CH₃SO₃Li, CH₂FSO₃Li, CHF₂SO₃Li,CF₃SO₃Li, CF₃CF₂SO₃Li, CF₃CF₂CF₂SO₃Li, and CF₃CF₂CF₂CF₂SO₃Li;

lithium imide salts such as LiN(FCO)₂, LiN(FCO) (FSO₂), LiN(FSO₂)₂,LiN(FSO₂) (CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithium cyclic1,2-perfluoroethanedisulfonylimide, lithium cyclic1,3-perfluoropropanedisulfonylimide, and LiN(CF₃SO₂) (C₄F₉SO₂);

lithium methide salts such as LiC(FSO₂)₃, LiC(CF₃SO₂)₃, andLiC(C₂F₅SO₂)₃;

lithium oxalatoborates such as lithium difluorooxalatoborate and lithiumbis(oxalato)borate;

lithium oxalatophosphates such as lithium tetrafluorooxalatophosphate,lithium difluorobis(oxalato)phosphate, and lithiumtris(oxalato)phosphate; and

fluorine-containing organic lithium salts such as salts represented bythe formula: LiPF_(a)(C_(n)F_(2n+1))_(6−a) (wherein a is an integer of 0to 5; and n is an integer of 1 to 6) (e.g., LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂),LiPF₄(CF₃SO₂)₂, LiPF₄ (C₂F₅SO₂)₂, LiBF₃CF₃, LiBF₃C₂F₅, LiBF₃C₃F₇,LiBF₂(CF₃)₂, LiBF₂(C₂F₅)₂, LiBF₂(CF₃SO₂)₂, and LiBF₂(C₂F₅SO₂)₂.

In order to give a low resistance and to achieve an effect of improvingcharacteristics such as high-rate charge and discharge characteristics,high-temperature storage characteristics, and cycle characteristics,particularly preferred among these are LiPF₆, LiBF₄, FSO₃Li, CF₃SO₃Li,LiN(FSO₂)₂, LiN(CF₃SO₂)₂, lithium cyclic 1,2-perfluoroethane disulfonylimide, lithium cyclic 1,3-perfluoropropane disulfonyl imide, LiC(FSO₂)₃,LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, lithium bis(oxalato)borate, lithiumdifluorooxalatoborate, lithium tetrafluorooxalatophosphate, lithiumdifluorobis(oxalato)phosphate, LiBF₃CF₃, LiBF₃C₂F₅, LiPF₃(CF₃)₃, andLiPF₃(C₂F₅)₃.

These lithium salts may be used alone or in combination of two or more.In combination use of two or more thereof, preferred examples thereofinclude a combination of LiPF₆ and LiBF₄ and a combination of LiPF₆ andFSO₃Li, which has an effect of improving the load characteristics andthe cycle characteristics.

In this case, LiBF₄ or FSO₃Li may be used in any amount that does notsignificantly impair the effects of the invention in 100% by mass of thewhole electrolyte solution. The amount thereof is usually 0.01% by massor more, preferably 0.5% by mass or more, while usually 30% by mass orless, preferably 15% by mass or less, relative to the electrolytesolution of the invention.

In another example, an inorganic lithium salt and an organic lithiumsalt are used in combination. Such a combination has an effect ofreducing deterioration due to high-temperature storage. The organiclithium salt is preferably CF₃SO₃Li, LiN(FSO₂)₂, LiN(FSO₂) (CF₃SO₂),LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, lithium cyclic1,2-perfluoroethanedisulfonylimide, lithium cyclic1,3-perfluoropropanedisulfonylimide, LiC(FSO₂)₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, lithium bis(oxalato)borate, lithiumdifluorooxalatoborate, lithium tetrafluorooxalatophosphate, lithiumdifluorobis(oxalato)phosphate, LiBF₃CF₃, LiBF₃C₂F₅, LiPF₃(CF₃)₃,LiPF₃(C₂F₅)₃, or the like. In this case, the proportion of the organiclithium salt is preferably 0.1% by mass or more, particularly preferably0.5% by mass or more, while preferably 30% by mass or less, particularlypreferably 20% by mass or less, in 100% by mass of the whole electrolytesolution.

The lithium salt in the electrolyte solution may have any concentrationthat does not impair the effects of the invention. In order to make theelectric conductivity of the electrolyte solution within a favorablerange and to ensure good battery performance, the lithium in theelectrolyte solution preferably has a total mole concentration of 0.3mol/L or higher, more preferably 0.5 mol/L or higher, still morepreferably 0.7 mol/L or higher, while preferably 4.0 mol/L or lower,more preferably 2.0 mol/L or lower, still more preferably 1.5 mol/L orlower.

Too low a total mole concentration of lithium may cause insufficientelectric conductivity of the electrolyte solution, while too high aconcentration may cause a viscosity increase and then reduce theelectric conductivity, impairing the battery performance.

The electrolyte salt in the electrolyte solution for an electric doublelayer capacitor is preferably an ammonium salt.

Examples of the ammonium salt include the following salts (IIa) to(IIe).

(IIa) Tetraalkyl Quaternary Ammonium Salts

Preferred examples thereof include tetraalkyl quaternary ammonium saltsrepresented by the following formula (IIa):

(wherein R^(1a), R^(2a), R^(3a), and R^(4a) are the same as or differentfrom each other, and are each a C1-C6 alkyl group optionally containingan ether bond; and X⁻ is an anion). In order to improve the oxidationresistance, any or all of the hydrogen atoms in the ammonium salt arealso preferably replaced by a fluorine atom and/or a C1-C4fluorine-containing alkyl group.

Preferred specific examples thereof include tetraalkyl quaternaryammonium salts represented by the following formula (IIa-1):

[Chem. 34]

(R^(1a))_(x)(R^(2a))_(y)N^(⊕)X^(⊖)  (IIa-1)

wherein R^(1a), R^(2a), and X⁻ are defined in the same manner asdescribed above; x and y are the same as or different from each other,and are each an integer of 0 to 4 with x+y=4, and

alkyl ether group-containing trialkyl ammonium salts represented by thefollowing formula (IIa-2):

wherein R^(5a) is a C1-C6 alkyl group; R^(6a) is a C1-C6 divalenthydrocarbon group; R^(7a) is a C1-C4 alkyl group; z is 1 or 2; and X⁻ isan anion.

Introduction of an alkyl ether group enables reduction in viscosity.

The anion X⁻ may be either an inorganic anion or an organic anion.Examples of the inorganic anion include AlCl₄ ⁻, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻,TaF₆ ⁻, I⁻, and SbF₆ ⁻. Examples of the organic anion include abis(oxalato)borate anion, a difluoro(oxalato)borate anion, atetrafluoro(oxalato)phosphate anion, a difluorobis(oxalato)phosphateanion, CF₃COO⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, and (C₂F₅SO₂)₂N⁻.

In order to achieve good oxidation resistance and ionic dissociation,BF₄, PF₆, AsF₆, and SbF₆ are preferred.

Preferred specific examples of the tetraalkyl quaternary ammonium saltsto be used include Et₄NBF₄, Et₄NClO₄, Et₄NPF₆, Et₄NAsF₆, Et₄NSbF₆,Et₄NCF₃SO₃, Et₄N(CF₃SO₂)₂N, Et₄NC₄F₉SO₃, Et₃MeNBF₄, Et₃MeNClO₄,Et₃MeNPF₆, Et₃MeNAsF₆, Et₃MeNSbF₆, Et₃MeNCF₃SO₃, Et₃MeN(CF₃SO₂)₂N, andEt₃MeNC₄F₉SO₃. In particular, Et₄NBF₄, Et₄NPF₆, Et₄NSbF₆, Et₄NAsF₆,Et₃MeNBF₄, and an N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium saltmay be mentioned as examples.

(IIb) Spirocyclic Bipyrrolidinium Salts Preferred examples thereofinclude spirocyclic bipyrrolidinium salts represented by the followingformula (IIb-1):

wherein R^(8a) and R^(9a) are the same as or different from each other,and are each a C1-C4 alkyl group; X⁻ is an anion; n1 is an integer of 0to 5; and n2 is an integer of 0 to 5,

spirocyclic bipyrrolidinium salts represented by the following formula(IIb-2):

wherein R^(10a) and R^(11a) are the same as or different from eachother, and are each a C1-C4 alkyl group; X⁻ is an anion; n3 is aninteger of 0 to 5; and n4 is an integer of 0 to 5, and

spirocyclic bipyrrolidinium salts represented by the following formula(IIb-3):

wherein R^(12a) and R^(13a) are the same as or different from eachother, and are each a C1-C4 alkyl group; X⁻ is an anion; n5 is aninteger of 0 to 5; and n6 is an integer of 0 to 5.

In order to improve the oxidation resistance, any or all of the hydrogenatoms in the spirocyclic bipyrrolidinium salt are also preferablyreplaced by a fluorine atom and/or a C1-C4 fluorine-containing alkylgroup.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa). In order to achieve good dissociation anda low internal resistance under high voltage, BF₄ ⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻,or (C₂F₅SO₂)₂N⁻ is particularly preferred.

For example, those represented by the following formulae:

may be mentioned as preferred specific examples of the spirocyclicbipyrrolidinium salts.

These spirocyclic bipyrrolidinium salts are excellent in solubility in asolvent, oxidation resistance, and ion conductivity.

(IIc) Imidazolium Salts

Preferred examples thereof include imidazolium salts represented by thefollowing formula (IIc):

(wherein R^(14a) and R^(15a) are the same as or different from eachother, and are each a C1-C6 alkyl group; and X⁻ is an anion). In orderto improve the oxidation resistance, any or all of the hydrogen atoms inthe imidazolium salt are also preferably replaced by a fluorine atomand/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, one represented by the following formula:

may be mentioned as a preferred specific example thereof.

This imidazolium salt is excellent in that it has low viscosity and goodsolubility.

(IId) N-Alkylpyridinium Salts

Preferred examples thereof include N-alkylpyridinium salts representedby the following formula (IId):

(wherein R^(16a) is a C1-C6 alkyl group; and X⁻ is an anion). In orderto improve the oxidation resistance, any or all of the hydrogen atoms inthe N-alkylpyridinium salt are also preferably replaced by a fluorineatom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, those represented by the following formulae:

may be mentioned as preferred specific examples thereof.

These N-alkylpyridinium salts are excellent in that they have lowviscosity and good solubility.

(IIe) N,N-dialkylpyrrolidinium Salts

Preferred examples thereof include N,N-dialkylpyrrolidinium saltsrepresented by the following formula (IIe):

wherein R^(17a) and R^(18a) are the same as or different from eachother, and are each a C1-C6 alkyl group; and X⁻ is an anion.

In order to improve the oxidation resistance, any or all of the hydrogenatoms in the N,N-dialkylpyrrolidinium salt are also preferably replacedby a fluorine atom and/or a C1-C4 fluorine-containing alkyl group.

Preferred specific examples of the anion X⁻ are the same as thosementioned for the salts (IIa).

For example, those represented by the following formulae:

may be mentioned as preferred specific examples thereof.

These N,N-dialkylpyrrolidinium salts are excellent in that they have lowviscosity and good solubility.

Preferred among these ammonium salts are those represented by theformula (IIa), (IIb), or (IIc) because they can have good solubility,oxidation resistance, and ion conductivity. More preferred are thoserepresented by the following formulae:

wherein Me is a methyl group; Et is an ethyl group; and X⁻, x, and y aredefined in the same manner as in the formula (IIa-1).

A lithium salt may be used as an electrolyte salt for an electric doublelayer capacitor. Preferred examples thereof include LiPF₆, LiBF₄,LiN(FSO₂)₂, LiAsF₆, LiSbF₆, and LiN(SO₂C2H₅)₂.

In order to further increase the capacity, a magnesium salt may be used.Preferred examples of the magnesium salt include Mg(ClO₄)₂ andMg(OOC₂H₅)₂.

The ammonium salt serving as an electrolyte salt is preferably used at aconcentration of 0.7 mol/L or higher. The ammonium salt at aconcentration lower than 0.7 mol/L may cause not only poorlow-temperature characteristics but also high initial internalresistance. The concentration of the electrolyte salt is more preferably0.9 mol/L or higher.

In order to achieve good low-temperature characteristics, the upperlimit of the concentration is preferably 2.0 mol/L or lower, morepreferably 1.5 mol/L or lower.

In order to achieve excellent low-temperature characteristics, theammonium salt which is triethyl methyl ammonium tetrafluoroborate(TEMABF₄) is preferably used at a concentration of 0.7 to 1.5 mol/L.

Spirobipyrrolidinium tetrafluoroborate (SBPBF₄) is preferably used at aconcentration of 0.7 to 2.0 mol/L.

The electrolyte solution of the invention may contain at least oneselected from the group consisting of nitrile compounds represented byany of the following formulae (1a), (1b), and (1c):

(wherein R^(a) and R^(b) are each individually a hydrogen atom, a cyanogroup (CN), a halogen atom, an alkyl group, or a group obtained byreplacing at least one hydrogen atom of an alkyl group with a halogenatom; and n is an integer of 1 to 10);

(wherein R^(c) is a hydrogen atom, a halogen atom, an alkyl group, agroup obtained by replacing at least one hydrogen atom of an alkyl groupwith a halogen atom, or a group represented by NC—R^(c1)—X^(c1)—(wherein R^(c1) is an alkylene group; and X^(c1) is an oxygen atom or asulfur atom); R^(d) and R^(e) are each individually a hydrogen atom, ahalogen atom, an alkyl group, or a group obtained by replacing at leastone hydrogen atom of an alkyl group with a halogen atom; and m is aninteger of 1 to 10); and

(wherein R^(f), R^(g), R^(h), and R^(i) are each individually a groupcontaining a cyano group (CN), a hydrogen atom (H), a halogen atom, analkyl group, or a group obtained by replacing at least one hydrogen atomof an alkyl group with a halogen atom; at least one of R^(f), R^(g),R^(h), or R^(i) is a group containing a cyano group; and 1 is an integerof 1 to 3).

This can improve the high-temperature storage characteristics of anelectrochemical device. The nitrile compounds may be used alone or inany combination of two or more at any ratio.

In the formula (1a), R^(a) and R^(b) are each individually a hydrogenatom, a cyano group (CN), a halogen atom, an alkyl group, or a groupobtained by replacing at least one hydrogen atom of an alkyl group witha halogen atom.

Examples of the halogen atom include a fluorine atom, a chlorine atom, abromine atom, and an iodine atom. Preferred among these is a fluorineatom.

The alkyl group is preferably a C1-C5 alkyl group. Specific examples ofthe alkyl group include a methyl group, an ethyl group, a propyl group,an isopropyl group, a butyl group, an isobutyl group, and a tert-butylgroup.

Examples of the group obtained by replacing at least one hydrogen atomof an alkyl group with a halogen atom include groups in which at leastone hydrogen atom of the above alkyl group with the above halogen atom.

When R^(a) and R^(b) are alkyl groups or groups obtained by replacing atleast one hydrogen atom of an alkyl group with a halogen atom, R^(a) andR^(b) may be bonded to each other to form a ring structure (e.g.,cyclohexane ring).

R^(a) and R^(b) are preferably hydrogen atoms or alkyl groups.

In the formula (1a), n is an integer of 1 to 10. When n is 2 or greater,all of n R^(a)s may be the same as each other, or at least some of themmay be different from each other. The same applies to R^(b). In theformula, n is preferably an integer of 1 to 7, more preferably aninteger of 2 to 5.

The nitrile compound represented by the formula (1a) is preferably adinitrile or a tricarbonitrile.

Specific examples of the dinitrile include malononitrile,succinonitrile, glutaronitrile, adiponitrile, pimelonitrile,suberonitrile, azelanitrile, sebaconitrile, undecanedinitrile,dodecanedinitrile, methyl malononitrile, ethyl malononitrile, isopropylmalononitrile, tert-butyl malononitrile, methyl succinonitrile,2,2-dimethyl succinonitrile, 2,3-dimethyl succinonitrile,2,3,3-trimethyl succinonitrile, 2,2,3,3-tetramethyl succinonitrile,2,3-diethyl-2,3-dimethyl succinonitrile, 2,2-diethyl-3,3-dimethylsuccinonitrile, bicyclohexyl-1,1-dicarbonitrile,bicyclohexyl-2,2-dicarbonitrile, bicyclohexyl-3,3-dicarbonitrile,2,5-dimethyl-2,5-hexanedicarbonitrile, 2,3-diisobutyl-2,3-dimethylsuccinonitrile, 2,2-diisobutyl-3,3-dimethyl succinonitrile, 2-methylglutaronitrile, 2,3-dimethyl glutaronitrile, 2,4-dimethylglutaronitrile, 2,2,3,3-tetramethyl glutaronitrile, 2,2,4,4-tetramethylglutaronitrile, 2,2,3,4-tetramethyl glutaronitrile, 2,3,3,4-tetramethylglutaronitrile, 1,4-dicyanopentane, 2,6-dicyanoheptane,2,7-dicyanooctane, 2,8-dicyanononane, and 1,6-dicyanodecane.Particularly preferred among these are succinonitrile, glutaronitrile,and adiponitrile.

Specific examples of the tricarbonitrile include pentanetricarbonitrile,propanetricarbonitrile, 1,3,5-hexanetricarbonitrile,1,3,5-cyclohexanetricarbonitrile, 1,3,6-hexanetricarbonitrile,heptanetricarbonitrile, 1,2,3-propanetricarbonitrile, and1,3,5-pentanetricarbonitrile. Particularly preferred are1,3,5-hexanetricarbonitrile and 1,3,6-hexanetricarbonitrile.

In the formula (1b), R^(c) is a hydrogen atom, a halogen atom, an alkylgroup, a group obtained by replacing at least one hydrogen atom of analkyl group with a halogen atom, or a group represented byNC—R^(c1)—X^(c1)— (wherein R^(c1) is an alkylene group; and X^(c1) is anoxygen atom or a sulfur atom), and R^(d) and R^(e) are each individuallya hydrogen atom, a halogen atom, an alkyl group, or a group obtained byreplacing at least one hydrogen atom of an alkyl group with a halogenatom.

Examples of the halogen atom, the alkyl group, and the group obtained byreplacing at least one hydrogen atom of an alkyl group with a halogenatom include those mentioned as examples for the formula (1a).

R^(c1) in NC—R^(c1)—X^(c1)— is an alkylene group. The alkylene group ispreferably a C1-C3 alkylene group.

R^(c), R^(d), and R^(e) are preferably each individually a hydrogenatom, a halogen atom, an alkyl group, or a group obtained by replacingat least one hydrogen atom of an alkyl group with a halogen atom.

At least one of R^(c), R^(d), or R^(e) is preferably a halogen atom or agroup obtained by replacing at least one hydrogen atom of an alkyl groupwith a halogen atom, more preferably a fluorine atom or a group obtainedby replacing at least one hydrogen atom of an alkyl group with afluorine atom.

When R^(d) and R^(e) are alkyl groups or groups obtained by replacing atleast one hydrogen atom of an alkyl group with a halogen atom, R^(d) andR^(e) may be bonded to each other to form a ring structure (e.g.,cyclohexane ring).

In the formula (1b), m is an integer of 1 to 10. When m is 2 or greater,all of m R^(d)s may be the same as each other, or at least some of themmay be different from each other. The same applies to R^(e). In theformula, m is preferably an integer of 2 to 7, more preferably aninteger of 2 to 5.

Examples of the nitrile compound represented by the formula (1b) includeacetonitrile, propionitrile, butyronitrile, isobutyronitrile,valeronitrile, isovaleronitrile, lauronitrile, 2-methyl butyronitrile,trimethyl acetonitrile, hexanenitrile, cyclopentanecarbonitrile,cyclohexanecarbonitrile, fluoroacetonitrile, difluoroacetonitrile,trifluoroacetonitrile, 2-fluoropropionitrile, 3-fluoropropionitrile,2,2-difluoropropionitrile, 2,3-difluoropropionitrile,3,3-difluoropropionitrile, 2,2,3-trifluoropropionitrile,3,3,3-trifluoropropionitrile, 3,3′-oxydipropionitrile,3,3′-thiodipropionitrile, and pentafluoropropionitrile. Particularlypreferred among these is 3,3,3-trifluoropropionitrile.

In the formula (1c), R^(f), R^(g), R^(h), and R^(i) are eachindividually a group containing a cyano group (CN), a hydrogen atom, ahalogen atom, an alkyl group, or a group obtained by replacing at leastone hydrogen atom of an alkyl group with a halogen atom.

Examples of the halogen atom, the alkyl group, and the group obtained byreplacing at least one hydrogen atom of an alkyl group with a halogenatom include those mentioned as examples for the formula (1a).

Examples of the group containing a cyano group include a cyano group andgroups obtained by replacing at least one hydrogen atom of an alkylgroup with a cyano group. Examples of the alkyl group in this caseinclude those mentioned as examples for the formula (1a).

At least one of R^(f), R^(g), R^(h), or R^(i) is a group containing acyano group. At least two selected from R^(f), R^(g), R^(h), and R^(i)are preferably groups containing a cyano group. More preferably, R^(h)and R^(i) are groups containing a cyano group. When R^(h) and R^(i) aregroups containing a cyano group, R^(f) and R^(g) are preferably hydrogenatoms.

In the formula (1c), l is an integer of 1 to 3. When l is 2 or greater,all of l R^(f)s may be the same as each other, or at least some of themmay be different from each other. The same applies to R^(g). Preferably,l is an integer of 1 or 2.

Examples of the nitrile compound represented by the formula (1c) include3-hexenedinitrile, mucononitrile, maleonitrile, fumaronitrile,acrylonitrile, methacrylonitrile, crotononitrile, 3-methylcrotononitrile, 2-methyl-2-butenenitrile, 2-pentenenitrile,2-methyl-2-pentenenitrile, 3-methyl-2-pentenenitrile, and2-hexenenitrile. Preferred are 3-hexenedinitrile and mucononitrile,particularly preferred is 3-hexenedinitrile.

The nitrile compound is preferably contained in an amount of 0.2 to 7%by mass relative to the electrolyte solution. This can further improvethe high-temperature storage characteristics and the safety of anelectrochemical device at high voltage. The lower limit of the sum ofthe amounts of the nitrile compounds is more preferably 0.3% by mass,still more preferably 0.5% by mass. The upper limit thereof is morepreferably 5% by mass, still more preferably 2% by mass, particularlypreferably 0.5% by mass.

The electrolyte solution of the invention may further contain a compound(B) represented by the formula (B).

The formula (B) is as follows.

In the formula,

A^(a+) is a metal ion, a hydrogen ion, or an onium ion;

a is an integer of 1 to 3;

b is an integer of 1 to 3;

p is b/a;

n²³ is an integer of 1 to 4;

n²¹ is an integer of 0 to 8;

n²² is 0 or 1;

Z²¹ is a transition metal or an element in group III, group IV, or groupV of the Periodic Table;

X²¹ is O, S, a C1-C10 alkylene group, a C1-C10 halogenated alkylenegroup, a C6-C20 arylene group, or a C6-C20 halogenated arylene group,with the alkylene group, the halogenated alkylene group, the arylenegroup, and the halogenated arylene group each optionally containing asubstituent and/or a hetero atom in the structure thereof, and when n²²is 1 and n²³ is 2 to 4, n²³ X²¹s optionally bind to each other;

L²¹ is a halogen atom, a cyano group, a C1-C10 alkyl group, a C1-C10halogenated alkyl group, a C6-C20 aryl group, a C6-C20 halogenated arylgroup, or —Z²³Y²³, with the alkylene group, the halogenated alkylenegroup, the arylene group, and the halogenated arylene group eachoptionally containing a substituent and/or a hetero atom in thestructure thereof, and when n²¹ is 2 to 8, n²¹ L²¹s optionally bind toeach other to form a ring;

Y²¹, Y²², and Z²³ are each individually O, S, NY²⁴, a hydrocarbon group,or a fluorinated hydrocarbon group;

Y²³ and Y²⁴ are each individually H, F, a C1-C10 alkyl group, a C1-C10halogenated alkyl group, a C6-C20 aryl group, or a C6-C20 halogenatedaryl group, with the alkyl group, the halogenated alkyl group, the arylgroup, and the halogenated aryl group each optionally containing asubstituent and/or a hetero atom in the structure thereof, and whenmultiple Y²³s or multiple Y²⁴s are present, they optionally bind to eachother to form a ring.

Examples of A^(a+) include a lithium ion, a sodium ion, a potassium ion,a magnesium ion, a calcium ion, a barium ion, a caesium ion, a silverion, a zinc ion, a copper ion, a cobalt ion, an iron ion, a nickel ion,a manganese ion, a titanium ion, a lead ion, a chromium ion, a vanadiumion, a ruthenium ion, an yttrium ion, lanthanoid ions, actinoid ions, atetrabutyl ammonium ion, a tetraethyl ammonium ion, a tetramethylammonium ion, a triethyl methyl ammonium ion, a triethyl ammonium ion, apyridinium ion, an imidazolium ion, a hydrogen ion, a tetraethylphosphonium ion, a tetramethyl phosphonium ion, a tetraphenylphosphonium ion, a triphenyl sulfonium ion, and a triethyl sulfoniumion.

In applications such as electrochemical devices, A^(a+) is preferably alithium ion, a sodium ion, a magnesium ion, a tetraalkyl ammonium ion,or a hydrogen ion, particularly preferably a lithium ion. The valence aof the cation A^(a+) is an integer of 1 to 3. If the valence a isgreater than 3, the crystal lattice energy is high and the compound (B)has difficulty in dissolving in a solvent. Thus, the valence a is morepreferably 1 when good solubility is needed. The valence b of the anionis also an integer of 1 to 3, particularly preferably 1. The constant pthat represents the ratio between the cation and the anion is naturallydefined by the ratio b/a between the valences a and b thereof.

Next, the ligands in the formula (B) are described. Herein, organic orinorganic groups binding to Z²¹ in the formula (B) are referred to asligands.

Z²¹ is preferably Al, B, V, Ti, Si, Zr, Ge, Sn, Cu, Y, Zn, Ga, Nb, Ta,Bi, P, As, Sc, Hf, or Sb, more preferably Al, B, or P.

X²¹ is O, S, a C1-C10 alkylene group, a C1-C10 halogenated alkylenegroup, a C6-C20 arylene group, or a C6-C20 halogenated arylene group.These alkylene groups and arylene groups each may have a substituentand/or a hetero atom in the structure. Specifically, instead of ahydrogen atom in the alkylene group or the arylene group, the structuremay have a halogen atom, a linear or cyclic alkyl group, an aryl group,an alkenyl group, an alkoxy group, an aryloxy group, a sulfonyl group,an amino group, a cyano group, a carbonyl group, an acyl group, an amidegroup, or a hydroxy group as a substituent; or, instead of a carbon atomin the alkylene or the arylene, the structure may have nitrogen, sulfur,or oxygen introduced therein. When n²² is 1 and n²³ is 2 to 4, n²³ X²¹smay bind to each other. One such example is a ligand such asethylenediaminetetraacetic acid.

L²¹ is a halogen atom, a cyano group, a C1-C10 alkyl group, a C1-C10halogenated alkyl group, a C6-C20 aryl group, a C6-C20 halogenated arylgroup, or —Z²³Y²³ (Z²³ and Y²³ will be described later). Similar to X²¹,the alkyl groups and the aryl groups each may have a substituent and/ora hetero atom in the structure, and when n²¹ is 2 to 8, n²¹ L²¹s maybind to each other to form a ring. L²¹ is preferably a fluorine atom ora cyano group. This is because a fluorine atom can improve thesolubility and the degree of dissociation of a salt of an anioncompound, thereby improving the ion conductivity. This is also because afluorine atom can improve the oxidation resistance, reducing occurrenceof side reactions.

Y²¹, Y²², and Z²³ are each individually O, S, NY²⁴, a hydrocarbon group,or a fluorinated hydrocarbon group. Y²¹ and Y²² are each preferably O,S, or NY²⁴, more preferably O. The compound (B) characteristically has abond between Y²¹ and Z²¹ and a bond between Y²² and Z²¹ in the sameligand. Such a ligand forms a chelate structure with Z²¹. The effect ofthis chelate improves the heat resistance, the chemical stability, andthe hydrolysis resistance of this compound. The constant n²² of theligand is 0 or 1. In particular, n²² is preferably 0 because the chelatering becomes a five-membered ring, leading to the most strongly exertedchelate effect and improved stability.

The term “fluorinated hydrocarbon group” as used herein means ahydrocarbon group in which at least one hydrogen atom is replaced by afluorine atom.

Y²³ and Y²⁴ are each individually H, F, a C1-C10 alkyl group, a C1-C10halogenated alkyl group, a C6-C20 aryl group, or a C6-C20 halogenatedaryl group. These alkyl groups and aryl groups each may contain asubstituent or a hetero atom in the structure. When multiple Y²³s ormultiple Y²⁴s are present, they may bind to each other to form a ring.

The constant n²³ relating to the number of the aforementioned ligands isan integer of 1 to 4, preferably 1 or 2, more preferably 2. The constantn²¹ relating to the number of the aforementioned ligands is an integerof 0 to 8, preferably an integer of 0 to 4, more preferably 0, 2, or 4.In addition, when n²³ is 1, n²¹ is preferably 2; and when n²³ is 2, n²¹is preferably 0.

In the formula (B), the alkyl group, the halogenated alkyl group, thearyl group, and the halogenated aryl group include those having anyother functional groups such as branches, hydroxy groups, and etherbonds.

The compound (B) is preferably a compound represented by the followingformula:

(wherein A^(a+), a, b, p, n²¹, Z²¹, and L²¹ are defined in the samemanner as described above), or a compound represented by the followingformula:

(wherein A^(a+), a, b, p, n²¹, Z²¹, and L²¹ are defined in the samemanner as described above).

The compound (B) may be a lithium (oxalato)borate salt. Examples thereofinclude lithium bis(oxalato)borate (LIBOB) represented by the followingformula:

and lithium difluoro(oxalato)borate (LIDFOB) represented by thefollowing formula:

Examples of the compound (B) also include lithiumdifluoro(oxalato)phosphanite (LIDFOP) represented by the followingformula:

lithium tetrafluoro(oxalato)phosphanite (LITFOP) represented by thefollowing formula:

and lithium bis(oxalato)difluorophosphanite represented by the followingformula:

In order to achieve much better cycle characteristics, the compound (B)is preferably contained in an amount of 0.001% by mass or more, morepreferably 0.01% by mass or more, while preferably 10% by mass or less,more preferably 3% by mass or less, relative to the solvent.

The electrolyte solution of the invention may include polyethylene oxidethat has a weight average molecular weight of 2000 to 4000 and has —OH,—OCOOH, or —COOH at an end.

The presence of such a compound can improve the stability at theinterfaces with the respective electrodes, improving the characteristicsof an electrochemical device.

Examples of the polyethylene oxide include polyethylene oxide monool,polyethylene oxide carboxylate, polyethylene oxide diol, polyethyleneoxide dicarboxylate, polyethylene oxide triol, and polyethylene oxidetricarboxylate. These may be used alone or in combination of two ormore.

In order to achieve better characteristics of an electrochemical device,preferred are a mixture of polyethylene oxide monool and polyethyleneoxide diol and a mixture of polyethylene carboxylate and polyethylenedicarboxylate.

The polyethylene oxide having too small a weight average molecularweight may be easily oxidatively decomposed. The weight averagemolecular weight is more preferably 3000 to 4000.

The weight average molecular weight can be determined by gel permeationchromatography (GPC) in polystyrene equivalent.

The amount of the polyethylene oxide is preferably 1×10⁻⁶ to 1×10⁻²mol/kg in the electrolyte solution. Too large an amount of thepolyethylene oxide may cause poor characteristics of an electrochemicaldevice.

The amount of the polyethylene oxide is more preferably 5×10⁻⁶ mol/kg ormore.

The electrolyte solution of the invention may further contain othercomponents such as an unsaturated cyclic carbonate, an overchargeinhibitor, and a known different aid. This can reduce impairment of thecharacteristics of an electrochemical device.

Examples of the unsaturated cyclic carbonate include vinylenecarbonates, ethylene carbonates substituted with a substituent thatcontains an aromatic ring, a carbon-carbon double bond, or acarbon-carbon triple bond, phenyl carbonates, vinyl carbonates, allylcarbonates, and catechol carbonates.

Examples of the vinylene carbonates include vinylene carbonate, methylvinylene carbonate, 4,5-dimethyl vinylene carbonate, phenyl vinylenecarbonate, 4,5-diphenyl vinylene carbonate, vinyl vinylene carbonate,4,5-divinyl vinylene carbonate, allyl vinylene carbonate, 4,5-diallylvinylene carbonate, 4-fluorovinylene carbonate, 4-fluoro-5-methylvinylene carbonate, 4-fluoro-5-phenyl vinylene carbonate,4-fluoro-5-vinyl vinylene carbonate, and 4-allyl-5-fluorovinylenecarbonate.

Specific examples of the ethylene carbonates substituted with asubstituent that contains an aromatic ring, a carbon-carbon double bond,or a carbon-carbon triple bond include vinyl ethylene carbonate,4,5-divinyl ethylene carbonate, 4-methyl-5-vinyl ethylene carbonate,4-allyl-5-vinyl ethylene carbonate, ethynyl ethylene carbonate,4,5-diethynyl ethylene carbonate, 4-methyl-5-ethynyl ethylene carbonate,4-vinyl-5-ethynyl ethylene carbonate, 4-allyl-5-ethynyl ethylenecarbonate, phenyl ethylene carbonate, 4,5-diphenyl ethylene carbonate,4-phenyl-5-vinyl ethylene carbonate, 4-allyl-5-phenyl ethylenecarbonate, allyl ethylene carbonate, 4,5-diallyl ethylene carbonate,4-methyl-5-allyl ethylene carbonate, 4-methylene-1,3-dioxolan-2-one, and4,5-dimethylene-1,3-dioxolan-2-one.

The unsaturated cyclic carbonate is preferably vinylene carbonate,methyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, vinylvinylene carbonate, 4,5-vinyl vinylene carbonate, allyl vinylenecarbonate, 4,5-diallyl vinylene carbonate, vinyl ethylene carbonate,4,5-divinyl ethylene carbonate, 4-methyl-5-vinyl ethylene carbonate,allyl ethylene carbonate, 4,5-diallyl ethylene carbonate,4-methyl-5-allyl ethylene carbonate, 4-allyl-5-vinyl ethylene carbonate,ethynyl ethylene carbonate, 4,5-diethynyl ethylene carbonate,4-methyl-5-ethynyl ethylene carbonate, and 4-vinyl-5-ethynyl ethylenecarbonate. In order to form a more stable interface protecting film,particularly preferred are vinylene carbonate, vinyl ethylene carbonate,and ethynyl ethylene carbonate.

The unsaturated cyclic carbonate may have any molecular weight that doesnot significantly impair the effects of the invention. The molecularweight is preferably 80 or higher and 250 or lower. The unsaturatedcyclic carbonate having a molecular weight within this range can easilyensure its solubility in the electrolyte solution and can easily lead tosufficient achievement of the effects of the invention. The molecularweight of the unsaturated cyclic carbonate is more preferably 85 orhigher, while more preferably 150 or lower.

The unsaturated cyclic carbonate may be produced by any productionmethod, and may be produced by a known method selected as appropriate.

The unsaturated cyclic carbonates may be used alone or in anycombination of two or more at any ratio.

The unsaturated cyclic carbonate may be contained in any amount thatdoes not significantly impair the effects of the invention. The amountof the unsaturated cyclic carbonate is preferably 0.001% by mass ormore, more preferably 0.01% by mass or more, still more preferably 0.1%by mass or more, of 100% by mass of the solvent in the invention. Theamount is preferably 5% by mass or less, more preferably 4% by mass orless, still more preferably 3% by mass or less. The unsaturated cycliccarbonate in an amount within the above range allows an electrochemicaldevice containing the electrolyte solution to easily exhibit an effectof improving the cycle characteristics, and can easily avoid a situationwith reduced high-temperature storage characteristics, generation of alarge amount of gas, and a reduced discharge capacity retention.

In addition to the aforementioned non-fluorinated unsaturated cycliccarbonates, a fluorinated unsaturated cyclic carbonate may also suitablybe used as an unsaturated cyclic carbonate.

The fluorinated unsaturated cyclic carbonate is a cyclic carbonatecontaining an unsaturated bond and a fluorine atom. The number offluorine atoms in the fluorinated unsaturated cyclic carbonate may beany number that is 1 or greater. The number of fluorine atoms is usually6 or smaller, preferably 4 or smaller, most preferably 1 or 2.

Examples of the fluorinated unsaturated cyclic carbonate includefluorinated vinylene carbonate derivatives and fluorinated ethylenecarbonate derivatives substituted with a substituent containing anaromatic ring or a carbon-carbon double bond.

Examples of the fluorinated vinylene carbonate derivatives include4-fluorovinylene carbonate, 4-fluoro-5-methyl vinylene carbonate,4-fluoro-5-phenyl vinylene carbonate, 4-allyl-5-fluorovinylenecarbonate, and 4-fluoro-5-vinyl vinylene carbonate.

Examples of the fluorinated ethylene carbonate derivatives substitutedwith a substituent containing an aromatic ring or a carbon-carbon doublebond include 4-fluoro-4-vinyl ethylene carbonate, 4-fluoro-4-allylethylene carbonate, 4-fluoro-5-vinyl ethylene carbonate,4-fluoro-5-allyl ethylene carbonate, 4,4-difluoro-4-vinyl ethylenecarbonate, 4,4-difluoro-4-allyl ethylene carbonate, 4,5-difluoro-4-vinylethylene carbonate, 4,5-difluoro-4-allyl ethylene carbonate,4-fluoro-4,5-divinyl ethylene carbonate, 4-fluoro-4,5-diallyl ethylenecarbonate, 4,5-difluoro-4,5-divinyl ethylene carbonate,4,5-difluoro-4,5-diallyl ethylene carbonate, 4-fluoro-4-phenyl ethylenecarbonate, 4-fluoro-5-phenyl ethylene carbonate, 4,4-difluoro-5-phenylethylene carbonate, and 4,5-difluoro-4-phenyl ethylene carbonate.

In order to form a stable interface protecting film, more preferred asthe fluorinated unsaturated cyclic carbonate are 4-fluorovinylenecarbonate, 4-fluoro-5-methyl vinylene carbonate, 4-fluoro-5-vinylvinylene carbonate, 4-allyl-5-fluorovinylene carbonate, 4-fluoro-4-vinylethylene carbonate, 4-fluoro-4-allyl ethylene carbonate,4-fluoro-5-vinyl ethylene carbonate, 4-fluoro-5-allyl ethylenecarbonate, 4,4-difluoro-4-vinyl ethylene carbonate, 4,4-difluoro-4-allylethylene carbonate, 4,5-difluoro-4-vinyl ethylene carbonate,4,5-difluoro-4-allyl ethylene carbonate, 4-fluoro-4,5-divinyl ethylenecarbonate, 4-fluoro-4,5-diallyl ethylene carbonate,4,5-difluoro-4,5-divinyl ethylene carbonate, and4,5-difluoro-4,5-diallyl ethylene carbonate.

The fluorinated unsaturated cyclic carbonate may have any molecularweight that does not significantly impair the effects of the invention.The molecular weight is preferably 50 or higher and 250 or lower. Thefluorinated unsaturated cyclic carbonate having a molecular weightwithin this range can easily ensure the solubility of the fluorinatedunsaturated cyclic carbonate in the electrolyte solution.

The fluorinated unsaturated cyclic carbonate may be produced by anymethod, and may be produced by any known method selected as appropriate.The molecular weight is more preferably 100 or higher and morepreferably 200 or lower.

The fluorinated unsaturated cyclic carbonates may be used alone or inany combination of two or more at any ratio. The fluorinated unsaturatedcyclic carbonate may be contained in any amount that does notsignificantly impair the effects of the invention. The amount of thefluorinated unsaturated cyclic carbonate is usually preferably 0.01% bymass or more, more preferably 0.1% by mass or more, still morepreferably 0.2% by mass or more, while preferably 5% by mass or less,more preferably 4% by mass or less, still more preferably 3% by mass orless, of 100% by mass of the electrolyte solution. The fluorinatedunsaturated cyclic carbonate in an amount within this range allows anelectrochemical device containing the electrolyte solution to exhibit aneffect of sufficiently improving the cycle characteristics and caneasily avoid the situation with reduced high-temperature storagecharacteristics, generation of a large amount of gas, and a reduceddischarge capacity retention.

In order to effectively reduce burst or combustion of batteries in caseof overcharge, for example, of an electrochemical device containing theelectrolyte solution, the electrolyte solution of the invention maycontain an overcharge inhibitor.

Examples of the overcharge inhibitor include aromatic compounds such asbiphenyl, alkyl biphenyl, terphenyl, partially hydrogenated terphenyl,cyclohexyl benzene, t-butyl benzene, t-amyl benzene, diphenyl ether, anddibenzofuran; partially fluorinated aromatic compounds such as2-fluorobiphenyl, o-cyclohexyl fluorobenzene, and p-cyclohexylfluorobenzene; and fluoroanisole compounds such as 2,4-difluoroanisole,2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole.Preferred are aromatic compounds such as biphenyl, alkyl biphenyl,terphenyl, partially hydrogenated terphenyl, cyclohexyl benzene, t-butylbenzene, t-amyl benzene, diphenyl ether, and dibenzofuran. Thesecompounds may be used alone or in combination of two or more. Forcombination use of two or more compounds, in order to achieve goodbalance between the overcharge inhibiting characteristics and thehigh-temperature storage characteristics, preferred is a combination ofcyclohexyl benzene and t-butyl benzene or t-amyl benzene, or acombination of at least one oxygen-free aromatic compound selected frombiphenyl, alkyl biphenyl, terphenyl, partially hydrogenated terphenyl,cyclohexyl benzene, t-butyl benzene, t-amyl benzene, and the like and atleast one oxygen-containing aromatic compound selected from diphenylether, dibenzofuran, and the like.

The electrolyte solution of the invention may further contain a knowndifferent aid. Examples of the different aid include carbonate compoundssuch as erythritan carbonate, spiro-bis-dimethylene carbonate, andmethoxyethyl-methyl carbonate; carboxylic anhydrides such as succinicanhydride, glutaric anhydride, maleic anhydride, citraconic anhydride,glutaconic anhydride, itaconic anhydride, diglycolic anhydride,cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylicdianhydride, and phenylsuccinic anhydride; spiro compounds such as2,4,8,10-tetraoxaspiro[5.5]undecane and3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane; sulfur-containingcompounds such as ethylene sulfite, 1,3-propanesultone,1-fluoro-1,3-propanesultone, 2-fluoro-1,3-propanesultone,3-fluoro-1,3-propanesultone, 1-propene-1,3-sultone,1-fluoro-1-propene-1,3-sultone, 2-fluoro-1-propene-1,3-sultone,3-fluoro-1-propene-1,3-sultone, 1,4-butanesultone, 1-butene-1,4-sultone,3-butene-1,4-sultone, methyl fluorosulfonate, ethyl fluorosulfonate,methyl methanesulfonate, ethyl methanesulfonate, busulfan, sulfolene,diphenyl sulfone, N,N-dimethylmethanesulfonamide,N,N-diethylmethanesulfonamide, methyl vinyl sulfonate, ethyl vinylsulfonate, allyl vinyl sulfonate, propargyl vinyl sulfonate, methylallyl sulfonate, ethyl allyl sulfonate, allyl allyl sulfonate, propargylallyl sulfonate, and 1,2-bis(vinylsulfonyloxy)ethane;nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone,1-methyl-2-piperidone, 3-methyl-2-oxazolidinone,1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide;phosphorus-containing compounds such as trimethyl phosphite, triethylphosphite, triphenyl phosphite, trimethyl phosphate, triethyl phosphate,triphenyl phosphate, dimethyl methyl phosphonate, diethyl ethylphosphonate, dimethyl vinyl phosphonate, diethyl vinyl phosphonate,ethyl diethyl phosphonoacetate, methyl dimethyl phosphinate, ethyldiethyl phosphinate, trimethylphosphine oxide, and triethylphosphineoxide; hydrocarbon compounds such as heptane, octane, nonane, decane,and cycloheptane; and fluorine-containing aromatic compounds such asfluorobenzene, difluorobenzene, hexafluorobenzene, and benzotrifluoride.These compounds may be used alone or in combination of two or more.These aids can improve the capacity retention characteristics and thecycle characteristics after high-temperature storage.

The different aid may be used in any amount that does not significantlyimpair the effects of the invention. The amount of the different aid ispreferably 0.01% by mass or more and 5% by mass or less in 100% by massof the electrolyte solution. The different aid in an amount within thisrange can easily sufficiently exhibit the effects thereof and can easilyavoid the situation with reduction in battery characteristics such ashigh-load discharge characteristics. The amount of the different aid ismore preferably 0.1% by mass or more, still more preferably 0.2% by massor more, while more preferably 3% by mass or less, still more preferably1% by mass or less.

The electrolyte solution of the invention may further contain any ofadditives such as a cyclic carboxylate, an ether compound, anitrogen-containing compound, a boron-containing compound, anorganosilicon-containing compound, a fireproof agent (flame retardant),a surfactant, an additive for increasing the permittivity, and animprover for cycle characteristics or rate characteristics, to theextent that does not impair the effects of the invention.

Examples of the cyclic carboxylate include those having a carbon numberof 3 to 12 in total in the structural formula. Specific examples thereofinclude gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone,and epsilon-caprolactone. In order to improve the characteristics of anelectrochemical device owing to improvement in the degree ofdissociation of lithium ions, gamma-butyrolactone is particularlypreferred.

In general, the amount of the cyclic carboxylate as an additive ispreferably 0.1% by mass or more, more preferably 1% by mass or more, in100% by mass of the electrolyte solution. The cyclic carboxylate in anamount within this range can easily improve the electric conductivity ofthe electrolyte solution, improving the large-current dischargecharacteristics of an electrochemical device. The amount of the cycliccarboxylate is also preferably 10% by mass or less, more preferably 5%by mass or less. Such an upper limit may allow the electrolyte solutionto have a viscosity within an appropriate range, may make it possible toavoid a reduction in the electric conductivity, may reduce an increasein the resistance of the negative electrode, and may allow theelectrochemical device to have large-current discharge characteristicswithin a favorable range.

The cyclic carboxylate to be suitably used may also be a fluorinatedcyclic carboxylate (fluorine-containing lactone). Examples of thefluorine-containing lactone include fluorine-containing lactonesrepresented by the following formula (C):

wherein X¹⁵ to X²⁰ are the same as or different from each other, and areeach —H, —F, —Cl, —CH₃, or a fluorinated alkyl group; and at least oneof X¹⁵ to X²⁰ is a fluorinated alkyl group.

Examples of the fluorinated alkyl group for X¹⁵ to X²⁰ include —CFH₂,—CF₂H, —CF₃, —CH₂CF₃, —CF₂CF₃, —CH₂CF₂CF₃, and —CF(CF₃)₂. In order toachieve high oxidation resistance and an effect of improving the safety,—CH₂CF₃ and —CH₂CF₂CF₃ are preferred.

One of X¹⁵ to X²⁰ or a plurality thereof may be replaced by —H, —F, —Cl,—CH₃, or a fluorinated alkyl group only when at least one of X¹⁵ to X²⁰is a fluorinated alkyl group. In order to achieve good solubility of theelectrolyte salt, the number of substituents is preferably 1 to 3, morepreferably 1 or 2.

The substitution of the fluorinated alkyl group may be at any of theabove sites. In order to achieve a good synthesizing yield, thesubstitution site is preferably X¹⁷ and/or X¹⁸. In particular, X¹⁷ orX¹⁸ is preferably a fluorinated alkyl group, especially —CH₂CF₃ or—CH₂CF₂CF₃. The substituent for X¹⁵ to X²⁰ other than the fluorinatedalkyl group is —H, —F, —Cl, or CH₃. In order to achieve good solubilityof the electrolyte salt, —H is preferred.

In addition to those represented by the above formula, thefluorine-containing lactone may also be a fluorine-containing lactonerepresented by the following formula (D):

wherein one of A and B is CX²⁶X²⁷ (where X²⁶ and X²⁷ are the same as ordifferent from each other, and are each —H, —F, —Cl, —CF₃, —CH₃, or analkylene group in which a hydrogen atom is optionally replaced by ahalogen atom and which optionally contains a hetero atom in the chain)and the other is an oxygen atom; Rf¹² is a fluorinated alkyl group orfluorinated alkoxy group optionally containing an ether bond; X²¹ andX²² are the same as or different from each other, and are each —H, —F,—Cl, —CF₃, or CH₃; X²³ to X²⁵ are the same as or different from eachother, and are each —H, —F, —Cl, or an alkyl group in which a hydrogenatom is optionally replaced by a halogen atom and which optionallycontains a hetero atom in the chain; and n=0 or 1.

A preferred example of the fluorine-containing lactone represented bythe formula (D) is a 5-membered ring structure represented by thefollowing formula (E):

(wherein A, B, Rf¹², X²¹, X²², and X²³ are defined in the same manner asin the formula (D)) because it can be easily synthesized and can havegood chemical stability. Further, in relation to the combination of Aand B, fluorine-containing lactones represented by the following formula(F):

(wherein Rf¹², X²¹, X²², X²³, X²⁶, and X²⁷ are defined in the samemanner as in the formula (D)) and fluorine-containing lactonesrepresented by the following formula (G):

(wherein Rf¹², X²¹, X²², X²³, X²⁶, and X²⁷ are defined in the samemanner as in the formula (D)) may be mentioned.

In order to particularly achieve excellent characteristics such as highpermittivity and high withstand voltage, and to improve thecharacteristics of the electrolyte solution in the invention, forexample, to achieve good solubility of the electrolyte salt and toreduce the internal resistance well, those represented by the followingformulae:

may be mentioned.

The presence of a fluorinated cyclic carboxylate can lead to, forexample, effects of improving the ion conductivity, improving thesafety, and improving the stability at high temperature.

The ether compound is preferably a C3-C10 acyclic ether or a C3-C6cyclic ether.

Examples of the C3-C10 acyclic ether include diethyl ether, di-n-butylether, dimethoxymethane, methoxyethoxymethane, diethoxymethane,dimethoxyethane, methoxyethoxyethane, diethoxyethane, ethylene glycoldi-n-propyl ether, ethylene glycol di-n-butyl ether, and diethyleneglycol dimethyl ether.

Further, the ether compound may also suitably be a fluorinated ether.

An example of the fluorinated ether is a fluorinated ether (I)represented by the following formula (I):

Rf³—O—Rf⁴  (I)

(wherein Rf³ and Rf⁴ are the same as or different from each other, andare each a C1-C10 alkyl group or a C1-C10 fluorinated alkyl group; andat least one selected from Rf³ and Rf⁴ is a fluorinated alkyl group).The presence of the fluorinated ether (K) can improve theincombustibility of the electrolyte solution, as well as improve thestability and safety at high temperature under high voltage.

In the formula (I), at least one selected from Rf³ and Rf⁴ has only tobe a C1-C10 fluorinated alkyl group. In order to further improve theincombustibility and the stability and safety at high temperature underhigh voltage of the electrolyte solution, both Rf³ and Rf⁴ arepreferably C1-C10 fluorinated alkyl groups. In this case, Rf³ and Rf⁴may be the same as or different from each other.

Particularly preferably, Rf³ and Rf⁴ are the same as or different fromeach other, and Rf³ is a C3-C6 fluorinated alkyl group and Rf⁴ is aC2-C6 fluorinated alkyl group.

If the sum of the carbon numbers of Rf³ and Rf⁴ is too small, thefluorinated ether may have too low a boiling point. Too large a carbonnumber of Rf³ or Rf⁴ may cause low solubility of the electrolyte salt,may start to adversely affect the miscibility with other solvents, andmay cause high viscosity, resulting in poor rate characteristics. Inorder to achieve an excellent boiling point and rate characteristics,advantageously, the carbon number of Rf³ is 3 or 4 and the carbon numberof Rf⁴ is 2 or 3.

The fluorinated ether (I) preferably has a fluorine content of 20 to 75%by mass. The fluorinated ether (K) having a fluorine content within thisrange may lead to particularly excellent balance between thenon-flammability and the miscibility. The above range is also preferredfor good oxidation resistance and safety.

The lower limit of the fluorine content is more preferably 30% by mass,still more preferably 40% by mass, particularly preferably 55% by mass.The upper limit thereof is more preferably 70% by mass, still morepreferably 66% by mass.

The fluorine content of the fluorinated ether (I) is a value calculatedbased on the structural formula of the fluorinated ether (I) by thefollowing formula:

{(Number of fluorine atoms×19)/(molecular weight of fluorinatedether(I))}×100(%).

Examples of Rf³ include CF₃CF₂—, CF₃CH₂—, CF₂HCF₂—, CF₃CF₂CH₂—,CF₃CFHCF₂—, HCF₂CF₂CF₂—, HCF₂CF₂CH₂—, CF₃CF₂CH₂CH₂—, CF₃CFHCF₂CH₂—,HCF₂CF₂CF₂CF₂—, HCF₂CF₂CF₂CH₂—, HCF₂CF₂CH₂CH₂—, and HCF₂CF(CF₃)CH₂—.Examples of Rf⁴ include —CH₂CF₂CF₃, —CF₂CFHCF₃, —CF₂CF₂CF₂H,—CH₂CF₂CF₂H, —CH₂CH₂CF₂CF₃, —CH₂CF₂CFHCF₃, —CF₂CF₂CF₂CF₂H,—CH₂CF₂CF₂CF₂H, —CH₂CH₂CF₂CF₂H, —CH₂CF(CF₃)CF₂H, —CF₂CF₂H, —CH₂CF₂H, and—CF₂CH₃.

Specific examples of the fluorinated ether (I) includeHCF₂CF₂CH₂OCF₂CF₂H, CF₃CF₂CH₂OCF₂CF₂H, HCF₂CF₂CH₂OCF₂CFHCF₃,CF₃CF₂CH₂OCF₂CFHCF₃, C₆F₁₃OCH₃, C₆F₁₃OC₂H₅, C₈F₁₇OCH₃, C₈F₁₇OC₂H₅,CF₃CFHCF₂CH(CH₃) OCF₂CFHCF₃, HCF₂CF₂OCH(C₂H₅)₂, HCF₂CF₂OC₄H₉,HCF₂CF₂OCH₂CH(C₂H₅)₂, HCF₂CF₂OCH₂CH(CH₃)₂, HCF₂CF₂OC₃H₇, andCF₃CH₂OCH₂CH₂OCH₃.

In particular, those having HCF₂— or CF₃CFH— at one end or both ends canprovide a fluorinated ether (I) having excellent polarizability and ahigh boiling point. The boiling point of the fluorinated ether (I) ispreferably 67° C. to 120° C., more preferably 80° C. or higher, stillmore preferably 90° C. or higher.

Such a fluorinated ether (I) may include one or two or more ofCF₃CH₂OCF₂CFHCF₃, CF₃CF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CFHCF₃,HCF₂CF₂CH₂OCH₂CF₂CF₂H, CF₃CFHCF₂CH₂OCF₂CFHCF₃, HCF₂CF₂CH₂OCF₂CF₂H,CF₃CF₂CH₂OCF₂CF₂H, and the like.

The fluorinated ether (I) is preferably at least one selected from thegroup consisting of HCF₂CF₂CH₂OCF₂CFHCF₃ (boiling point: 106° C.),CF₃CF₂CH₂OCF₂CFHCF₃ (boiling point: 82° C.), HCF₂CF₂CH₂OCF₂CF₂H (boilingpoint: 92° C.), and CF₃CF₂CH₂OCF₂CF₂H (boiling point: 68° C.), morepreferably at least one selected from the group consisting ofHCF₂CF₂CH₂OCF₂CFHCF₃ (boiling point: 106° C.), and HCF₂CF₂CH₂OCF₂CF₂H(boiling point: 92° C.), because they can advantageously have a highboiling point and good miscibility with other solvents, and lead to goodsolubility of the electrolyte salt.

Examples of the C3-C6 cyclic ether include 1,3-dioxane,2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane, and fluorinatedcompounds thereof. Preferred are dimethoxymethane, diethoxymethane,ethoxymethoxymethane, ethylene glycol n-propyl ether, ethylene glycoldi-n-butyl ether, and diethylene glycol dimethyl ether because they canhave a high ability to solvate with lithium ions and improve the degreeof ion dissociation. Particularly preferred are dimethoxymethane,diethoxymethane, and ethoxymethoxymethane because they can have lowviscosity and give a high ion conductivity.

Examples of the nitrogen-containing compound include nitrile,fluorine-containing nitrile, carboxylic acid amide, fluorine-containingcarboxylic acid amide, sulfonic acid amide, and fluorine-containingsulfonic acid amide. Also, 1-methyl-2-pyrrolidinone,1-methyl-2-piperidone, 3-methyl-2-oxazilidinone,1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide may be used. Thenitrile compounds represented by any of the formulae (1a), (1b), and(1c) are not included in the nitrogen-containing compound.

Examples of the boron-containing compound include borates such astrimethyl borate and triethyl borate, boric acid ethers, and alkylborates.

Examples of the organosilicon-containing compound include (CH₃)₄—Si and(CH₃)₃—Si—Si(CH₃)₃.

Examples of the fireproof agent (flame retardant) includeorganophosphates and phosphazene-based compounds. Examples of theorganophosphates include fluorine-containing alkyl phosphates,non-fluorine-containing alkyl phosphates, and aryl phosphates. In orderto achieve a flame retardant effect even at a small amount,fluorine-containing alkyl phosphates are particularly preferred.

Specific examples of the fluorine-containing alkyl phosphates includefluorine-containing dialkyl phosphates disclosed in JP H11-233141 A,cyclic alkyl phosphates disclosed in JP H11-283669 A, andfluorine-containing trialkyl phosphates.

Preferred examples of the fireproof agent (flame retardant) include(CH₃O)₃P═O, (CF₃CH₂O)₃P═O, (HCF₂CH₂O)₃P═O, (CF₃CF₂CH₂)₃P═O,(CF₃CH₂)₃P═O, and (HCF₂CF₂CH₂)₃P═O.

The surfactant may be any of cationic surfactants, anionic surfactants,nonionic surfactants, and amphoteric surfactants. In order to achievegood cycle characteristics and rate characteristics, the surfactant ispreferably one containing a fluorine atom.

Preferred examples of such a surfactant containing a fluorine atominclude fluorine-containing carboxylic acid salts represented by thefollowing formula (3):

Rf⁵COO⁻M⁺  (3)

(wherein Rf⁵ is a C3-C10 fluorine-containing alkyl group optionallycontaining an ether bond; M⁺ is Li⁺, Na⁺, K⁺, or NHR′₃ ⁺, wherein R′sare the same as or different from each other, and are each H or a C1-C3alkyl group), and fluorine-containing sulfonic acid salts represented bythe following formula (4):

Rf⁶SO₃₃ ⁻M⁺  (4)

(wherein Rf⁶ is a C3-C10 fluorine-containing alkyl group optionallycontaining an ether bond; M⁺ is Li⁺, Na⁺, K⁺, or NHR′₃ ⁺, wherein R′sare the same as or different from each other, and are each H or a C1-C3alkyl group).

In order to reduce the surface tension of the electrolyte solutionwithout impairing the charge and discharge cycle characteristics, theamount of the surfactant is preferably 0.01 to 2% by mass in theelectrolyte solution.

Examples of the additive for increasing the permittivity includesulfolane, methyl sulfolane, γ-butyrolactone, and γ-valerolactone.

Examples of the improver for cycle characteristics and ratecharacteristics include methyl acetate, ethyl acetate, tetrahydrofuran,and 1,4-dioxane.

The electrolyte solution of the invention may be combined with a polymermaterial and thereby formed into a gel-like (plasticized), gelelectrolyte solution.

Examples of such a polymer material include conventionally knownpolyethylene oxide and polypropylene oxide, and modified productsthereof (see JP H08-222270 A, JP 2002-100405 A); polyacrylate-basedpolymers, polyacrylonitrile, and fluororesins such as polyvinylidenefluoride and vinylidene fluoride-hexafluoropropylene copolymers (see JPH04-506726 T, JP H08-507407 T, JP H10-294131 A); and composites of anyof these fluororesins and any hydrocarbon resin (see JP H11-35765 A, JPH11-86630 A). In particular, polyvinylidene fluoride or a vinylidenefluoride-hexafluoropropylene copolymer is preferably used as a polymermaterial for a gel electrolyte.

The electrolyte solution of the invention may also contain an ionconductive compound disclosed in Japanese Patent Application No.2004-301934.

This ion conductive compound is an amorphous fluorine-containingpolyether compound having a fluorine-containing group at a side chainand is represented by the following formula (1-1):

A-(D)-B  (1-1)

wherein D is represented by the following formula (2-1):

-(D1)_(n)-(FAE)_(m)-(AE)_(p)-(Y)_(q)—  (2-1)

[wherein D1 is an ether unit containing a fluorine-containing ethergroup at a side chain and is represented by the following formula (2a):

(wherein Rf is a fluorine-containing ether group optionally containing acrosslinkable functional group; and R¹⁰ is a group or a bond that linksRf and the main chain);

FAE is an ether unit containing a fluorinated alkyl group at a sidechain and is represented by the following formula (2b):

(wherein Rfa is a hydrogen atom or a fluorinated alkyl group optionallycontaining a crosslinkable functional group; and R¹¹ is a group or abond that links Rfa and the main chain);

AE is an ether unit represented by the following formula (2c):

(wherein R¹³ is a hydrogen atom, an alkyl group optionally containing acrosslinkable functional group, an aliphatic cyclic hydrocarbon groupoptionally containing a crosslinkable functional group, or an aromatichydrocarbon group optionally containing a crosslinkable functionalgroup; and R¹² is a group or a bond that links R¹³ and the main chain);

Y is a unit containing at least one selected from the following formulae(2d-1) to (2d-3):

n is an integer of 0 to 200;

m is an integer of 0 to 200;

p is an integer of 0 to 10000;

q is an integer of 1 to 100;

n+m is not 0; and

the bonding order of D1, FAE, AE, and Y is not specified]; and

A and B are the same as or different from each other, and are each ahydrogen atom, an alkyl group optionally containing a fluorine atomand/or a crosslinkable functional group, a phenyl group optionallycontaining a fluorine atom and/or a crosslinkable functional group, a—COOH group, —OR (where R is a hydrogen atom or an alkyl groupoptionally containing a fluorine atom and/or a crosslinkable functionalgroup), an ester group, or a carbonate group, and when an end of D is anoxygen atom, A and B are each none of a —COOH group, —OR, an estergroup, and a carbonate group).

The electrolyte solution of the invention may further contain adifferent additive, if necessary. Examples of the different additiveinclude metal oxides and glass.

The electrolyte solution of the invention preferably has a hydrogenfluoride (HF) content of 5 to 300 ppm. The presence of HF can promoteformation of a film of the above additive. Too small an amount of HFtends to impair the ability to form a film on the negative electrode,impairing the characteristics of an electrochemical device. Too large anamount of HF tends to impair the oxidation resistance of the electrolytesolution due to the influence by HF. The electrolyte solution of theinvention, even when containing HF in an amount within the above range,causes no reduction in capacity recovery in high-temperature storage ofan electrochemical device.

The amount of HF is more preferably 10 ppm or more, still morepreferably 20 ppm or more. The amount of HF is also more preferably 150ppm or less, still more preferably 100 ppm or less, particularlypreferably 50 ppm or less.

The amount of HF can be determined by neutralization titration.

The electrolyte solution of the invention may be prepared by any methodusing the aforementioned components.

The electrolyte solution of the invention can give a low resistance toan electrochemical device while maintaining the cycle characteristics ofthe electrochemical device and can reduce generation of gas duringcycles although the electrolyte solution contains a fluorine-basedsolvent as a result of the presence of the compounds (1) and (2) in thesolvent.

The electrolyte solution of the invention can be suitably applied toelectrochemical devices such as lithium-ion secondary batteries andelectric double layer capacitors. An electrochemical device includingthe electrolyte solution of the invention is also one aspect of theinvention.

Examples of the electrochemical devices include lithium-ion secondarybatteries, capacitors (electric double-layer capacitors), radicalbatteries, solar cells (in particular, dye-sensitized solar cells), fuelcells, various electrochemical sensors, electrochromic elements,electrochemical switching elements, aluminum electrolytic capacitors,and tantalum electrolytic capacitors. Preferred are lithium-ionsecondary batteries and electric double-layer capacitors.

A module including the electrochemical device is also one aspect of theinvention. A module including the lithium-ion secondary battery is alsoone aspect of the invention.

The invention also relates to a lithium-ion secondary battery includingthe electrolyte solution of the invention. The lithium-ion secondarybattery of the invention may include a positive electrode, a negativeelectrode, and the above electrolyte solution.

<Positive Electrode>

The positive electrode includes a positive electrode active materiallayer containing a positive electrode active material and a currentcollector.

The positive electrode active material may be any material that canelectrochemically occlude and release lithium ions. For example, asubstance containing lithium and at least one transition metal ispreferred. Specific examples thereof include lithium-containingtransition metal complex oxides and lithium-containing transition metalphosphoric acid compounds. In particular, the positive electrode activematerial is preferably a lithium-containing transition metal complexoxide that generates high voltage.

The transition metal of the lithium-containing transition metal complexoxide is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, or the like. Specificexamples thereof include lithium-cobalt complex oxides such as LiCoO₂,lithium-nickel complex oxides such as LiNiO₂, lithium-manganese complexoxides such as LiMnO₂, LiMn₂O₄, and Li₂MnO₄, and those obtained bysubstituting some of transition metal atoms as main components of theselithium transition metal complex oxides with another element such as Na,K, B, F, Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, Nb,Mo, Sn, or W. Specific examples of those obtained by substitutioninclude LiNi_(0.5)Mn_(0.5)O₂, LiNi_(0.85)Co_(0.10)Al_(0.05)O₂,LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂, LiNi_(0.6)Co_(0.2)Mn_(0.2) O₂,LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂, LiNi_(0.45)Co_(0.10)Al_(0.45)O₂,LiMn_(1.8)Al_(0.2)O₄, and LiMn_(1.5)Ni_(0.5)O₄.

The lithium-containing transition metal complex oxide is preferably anyof LiMn_(1.5)Ni_(0.5)O₄, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂, andLiNi_(0.6)Co_(0.2)Mn_(0.2) O₂ each of which has a high energy densityeven at high voltage.

The transition metal of the lithium-containing transition metalphosphoric acid compound is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, orthe like. Specific examples thereof include iron phosphates such asLiFePO₄, Li₃Fe₂(PO₄)₃, and LiFeP₂O₇, cobalt phosphates such as LiCoPO₄,and those obtained by substituting some of transition metal atoms asmain components of these lithium transition metal phosphoric acidcompounds with another element such as Al, Ti, V, Cr, Mn, Fe, Co, Li,Ni, Cu, Zn, Mg, Ga, Zr, Nb, or Si.

Examples of the lithium-containing transition metal complex oxideinclude

lithium-manganese spinel complex oxides represented by the formula:Li_(a)Mn_(2-b)M¹ _(b)O₄ (wherein 0.9≤a; 0≤b≤1.5; and M¹ is at least onemetal selected from the group consisting of Fe, Co, Ni, Cu, Zn, Al, Sn,Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge),

lithium-nickel complex oxides represented by the formula: LiNi_(1-c)M²_(c)O₂ (wherein 0≤c≤0.5; and M² is at least one metal selected from thegroup consisting of Fe, Co, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr,B, Ga, In, Si, and Ge), and

lithium-cobalt complex oxides represented by the formula: LiCo_(1-d)M³_(d)O₂ (wherein 0≤d≤0.5; and M³ is at least one metal selected from thegroup consisting of Fe, Ni, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr,B, Ga, In, Si, and Ge).

In order to provide a high-power lithium-ion secondary battery having ahigh energy density, preferred is LiCoO₂, LiMnO₂, LiNiO₂, LiMn₂O₄,LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, or LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂.

Other examples of the positive electrode active material includeLiFePO₄, LiNi_(0.8)Co_(0.2)O₂, Li_(1.2)Fe_(0.4)Mn_(0.4)O₂,LiNi_(0.5)Mn_(0.5)O₂, and LiV₃O₆.

In order to improve the continuous charge characteristics, the positiveelectrode active material preferably contains lithium phosphate. Lithiumphosphate may be used in any manner, and is preferably used in admixturewith the positive electrode active material. The lower limit of theamount of lithium phosphate used is preferably 0.1% by mass or more,more preferably 0.3% by mass or more, still more preferably 0.5% by massor more, relative to the sum of the amounts of the positive electrodeactive material and lithium phosphate. The upper limit thereof ispreferably 10% by mass or less, more preferably 8% by mass or less,still more preferably 5% by mass or less.

To a surface of the positive electrode active material may be attached asubstance having a composition different from the positive electrodeactive material. Examples of the substance attached to the surfaceinclude oxides such as aluminum oxide, silicon oxide, titanium oxide,zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimonyoxide, and bismuth oxide; sulfates such as lithium sulfate, sodiumsulfate, potassium sulfate, magnesium sulfate, calcium sulfate, andaluminum sulfate; carbonates such as lithium carbonate, calciumcarbonate, and magnesium carbonate; and carbon.

Such a substance may be attached to a surface of the positive electrodeactive material by, for example, a method of dissolving or suspendingthe substance in a solvent, impregnating the solution or suspension intothe positive electrode active material, and drying the impregnatedmaterial; a method of dissolving or suspending a precursor of thesubstance in a solvent, impregnating the solution or suspension into thepositive electrode active material, and heating the material and theprecursor to cause a reaction therebetween; or a method of adding thesubstance to a precursor of the positive electrode active material andsimultaneously sintering the materials. In the case of attaching carbon,for example, a carbonaceous material in the form of activated carbon maybe mechanically attached to the surface afterward.

For the amount of the substance attached to the surface in terms of themass relative to the amount of the positive electrode active material,the lower limit thereof is preferably 0.1 ppm or more, more preferably 1ppm or more, still more preferably 10 ppm or more, while the upper limitthereof is preferably 20% or less, more preferably 10% or less, stillmore preferably 5% or less. The substance attached to the surface canreduce oxidation of the electrolyte solution on the surface of thepositive electrode active material, improving the battery life. Toosmall an amount of the substance may fail to sufficiently provide thiseffect. Too large an amount thereof may hinder the entrance and exit oflithium ions, increasing the resistance.

Particles of the positive electrode active material may have any shapeconventionally used, such as a bulky shape, a polyhedral shape, aspherical shape, an ellipsoidal shape, a plate shape, a needle shape, ora pillar shape. The primary particles may agglomerate to form secondaryparticles.

The positive electrode active material has a tap density of preferably0.5 g/cm³ or higher, more preferably 0.8 g/cm³ or higher, still morepreferably 1.0 g/cm³ or higher. The positive electrode active materialhaving a tap density below the lower limit may cause an increased amountof a dispersion medium required and increased amounts of a conductivematerial and a binder required in formation of the positive electrodeactive material layer, as well as limitation on the packing fraction ofthe positive electrode active material in the positive electrode activematerial layer, resulting in limitation on the battery capacity. Acomplex oxide powder having a high tap density enables formation of apositive electrode active material layer with a high density. The tapdensity is preferably as high as possible and has no upper limit, ingeneral. Still, too high a tap density may cause diffusion of lithiumions in the positive electrode active material layer with theelectrolyte solution serving as a diffusion medium to function as arate-determining step, easily impairing the load characteristics. Thus,the upper limit of the tap density is preferably 4.0 g/cm³ or lower,more preferably 3.7 g/cm³ or lower, still more preferably 3.5 g/cm³ orlower.

In the invention, the tap density is determined as a powder packingdensity (tap density) g/cm³ when 5 to 10 g of the positive electrodeactive material powder is packed into a 10-ml glass graduated cylinderand the cylinder is tapped 200 times with a stroke of about 20 mm.

The particles of the positive electrode active material have a mediansize d50 (or a secondary particle size when the primary particlesagglomerate to form secondary particles) of preferably 0.3 μm orgreater, more preferably 0.5 μm or greater, still more preferably 0.8 μmor greater, most preferably 1.0 μm or greater, while preferably 30 μm orsmaller, more preferably 27 μm or smaller, still more preferably 25 μmor smaller, most preferably 22 μm or smaller. The particles having amedian size below the lower limit may fail to provide a product with ahigh tap density. The particles having a median size greater than theupper limit may cause prolonged diffusion of lithium in the particles,impairing the battery performance and generating streaks in formation ofthe positive electrode for a battery, i.e., when the active material andcomponents such as a conductive material and a binder are formed intoslurry by adding a solvent and the slurry is applied in the form of afilm, for example. Mixing two or more positive electrode activematerials having different median sizes d50 can further improve theeasiness of packing in formation of the positive electrode.

In the invention, the median size d50 is determined using a known laserdiffraction/scattering particle size distribution analyzer. In the caseof using LA-920 (Horiba, Ltd.) as the particle size distributionanalyzer, the dispersion medium used in the measurement is a 0.1% bymass sodium hexametaphosphate aqueous solution and the measurementrefractive index is set to 1.24 after 5-minute ultrasonic dispersion.

When the primary particles agglomerate to form secondary particles, theaverage primary particle size of the positive electrode active materialis preferably 0.05 μm or greater, more preferably 0.1 μm or greater,still more preferably 0.2 μm or greater. The upper limit thereof ispreferably 5 μm or smaller, more preferably 4 μm or smaller, still morepreferably 3 μm or smaller, most preferably 2 μm or smaller. The primaryparticles having an average primary particle size greater than the upperlimit may have difficulty in forming spherical secondary particles,adversely affecting the powder packing. Further, such primary particlesmay have a greatly reduced specific surface area, highly possiblyimpairing the battery performance such as increasing the resistance. Incontrast, the primary particles having an average primary particle sizebelow the lower limit may usually be insufficiently grown crystals,causing poor charge and discharge reversibility, for example.

In the invention, the primary particle size is measured by scanningelectron microscopic (SEM) observation. Specifically, the primaryparticle size is determined as follows. A photograph at a magnificationof 10000× is first taken. Any 50 primary particles are selected and themaximum length between the left and right boundary lines of each primaryparticle is measured along the horizontal line. Then, the average valueof the maximum lengths is calculated, which is defined as the primaryparticle size.

The positive electrode active material has a BET specific surface areaof preferably 0.1 m²/g or larger, more preferably 0.2 m²/g or larger,still more preferably 0.3 m²/g or larger. The upper limit thereof ispreferably 50 m²/g or smaller, more preferably 40 m²/g or smaller, stillmore preferably 30 m²/g or smaller. The positive electrode activematerial having a BET specific surface area smaller than the above rangemay easily impair the battery performance. The positive electrode activematerial having a BET specific surface area larger than the above rangemay less easily have an increased tap density, easily causing adifficulty in applying the material in formation of the positiveelectrode active material layer.

In the invention, the BET specific surface area is defined by a valuedetermined by single point BET nitrogen adsorption utilizing a gas flowmethod using a surface area analyzer (e.g., fully automatic surface areameasurement device, Ohkura Riken Co., Ltd.), a sample pre-dried innitrogen stream at 150° C. for 30 minutes, and a nitrogen-helium gasmixture with the nitrogen pressure relative to the atmospheric pressurebeing accurately adjusted to 0.3.

When the lithium-ion secondary battery of the invention is used as alarge-size lithium-ion secondary battery for hybrid vehicles ordistributed generation, it needs to achieve high output. Thus, theparticles of the positive electrode active material preferably mainlycomposed of secondary particles.

The particles of the positive electrode active material preferablyinclude 0.5 to 7.0% by volume of fine particles having an averagesecondary particle size of 40 μm or smaller and having an averageprimary particle size of 1 μm or smaller. The presence of fine particleshaving an average primary particle size of 1 μm or smaller enlarges thecontact area with the electrolyte solution and enables more rapiddiffusion of lithium ions between the electrode and the electrolytesolution, improving the output performance of the battery.

The positive electrode active material may be produced by any usualmethod of producing an inorganic compound. In particular, a spherical orellipsoidal active material can be produced by various methods. Forexample, a material substance of transition metal is dissolved orcrushed and dispersed in a solvent such as water, and the pH of thesolution or dispersion is adjusted under stirring to form a sphericalprecursor. The precursor is recovered and, if necessary, dried. Then, aLi source such as LiOH, Li₂CO₃, or LiNO₃ is added thereto and themixture is sintered at high temperature, thereby providing an activematerial.

In production of the positive electrode, the aforementioned positiveelectrode active materials may be used alone or in any combination oftwo or more thereof having different compositions at any ratio.Preferred examples of the combination in this case include a combinationof LiCoO₂ and LiMn₂O₄ in which part of Mn may optionally be replaced bya different transition metal (e.g., LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂),and a combination with LiCoO₂ in which part of Co may optionally bereplaced by a different transition metal.

In order to achieve a high battery capacity, the amount of the positiveelectrode active material is preferably 50 to 99% by mass, morepreferably 80 to 99% by mass, of the positive electrode mixture. Theamount of the positive electrode active material in the positiveelectrode active material layer is preferably 80% by mass or more, morepreferably 82% by mass or more, particularly preferably 84% by mass ormore. The upper limit thereof is preferably 99% by mass or less, morepreferably 98% by mass or less. Too small an amount of the positiveelectrode active material in the positive electrode active materiallayer may cause an insufficient electric capacity. In contrast, toolarge an amount thereof may cause insufficient strength of the positiveelectrode.

The positive electrode mixture preferably further contains a binder, athickening agent, and a conductive material.

The binder may be any material that is safe against a solvent to be usedin production of the electrode and the electrolyte solution. Examplesthereof include resin polymers such as polyethylene, polypropylene,polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide,chitosan, alginic acid, polyacrylic acid, polyimide, cellulose, andnitro cellulose; rubbery polymers such as SBR (styrene-butadienerubber), isoprene rubber, butadiene rubber, fluoroelastomer, NBR(acrylonitrile-butadiene rubber), and ethylene-propylene rubber;styrene-butadiene-styrene block copolymers and hydrogenated productsthereof; thermoplastic elastomeric polymers such as EPDM(ethylene-propylene-diene terpolymers),styrene-ethylene-butadiene-styrene copolymers, styrene-isoprene-styreneblock copolymers and hydrogenated products thereof; soft resin polymerssuch as syndiotactic-1,2-polybutadiene, polyvinyl acetate,ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers;fluoropolymers such as polyvinylidene fluoride, polytetrafluoroethylene,vinylidene fluoride copolymers, and tetrafluoroethylene-ethylenecopolymers; and polymer compositions having ion conductivity of alkalimetal ions (especially, lithium ions). These may be used alone or in anycombination of two or more at any ratio.

The amount of the binder, which is expressed as the proportion of thebinder in the positive electrode active material layer, is usually 0.1%by mass or more, preferably 1% by mass or more, more preferably 1.5% bymass or more. The proportion is also usually 80% by mass or less,preferably 60% by mass or less, still more preferably 40% by mass orless, most preferably 10% by mass or less. Too low a proportion of thebinder may fail to sufficiently hold the positive electrode activematerial and cause insufficient mechanical strength of the positiveelectrode, impairing the battery performance such as cyclecharacteristics. In contrast, too high a proportion thereof may causereduction in battery capacity and conductivity.

Examples of the thickening agent include carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol,oxidized starch, monostarch phosphate, casein, and salts thereof. Theseagents may be used alone or in any combination of two or more at anyratio.

The proportion of the thickening agent relative to the active materialis usually 0.1% by mass or higher, preferably 0.2% by mass or higher,more preferably 0.3% by mass or higher, while usually 5% by mass orlower, preferably 3% by mass or lower, more preferably 2% by mass orlower. The thickening agent at a proportion lower than the above rangemay cause significantly poor easiness of application. The thickeningagent at a proportion higher than the above range may cause a lowproportion of the active material in the positive electrode activematerial layer, resulting in a low capacity of the battery and highresistance between the positive electrode active materials.

The conductive material may be any known conductive material. Specificexamples thereof include metal materials such as copper and nickel, andcarbon materials such as graphite, including natural graphite andartificial graphite, carbon black, including acetylene black, andamorphous carbon, including needle coke. These materials may be usedalone or in any combination of two or more at any ratio. The conductivematerial is used in an amount of usually 0.01% by mass or more,preferably 0.1% by mass or more, more preferably 1% by mass or more,while usually 50% by mass or less, preferably 30% by mass or less, morepreferably 15% by mass or less, in the positive electrode activematerial layer. The conductive material in an amount less than the aboverange may cause insufficient conductivity. In contrast, the conductivematerial in an amount more than the above range may cause a low batterycapacity.

The solvent for forming slurry may be any solvent that can dissolve ordisperse therein the positive electrode active material, the conductivematerial, and the binder, as well as a thickening agent used asappropriate. The solvent may be either an aqueous solvent or an organicsolvent. Examples of the aqueous medium include water and solventmixtures of an alcohol and water. Examples of the organic medium includealiphatic hydrocarbons such as hexane; aromatic hydrocarbons such asbenzene, toluene, xylene, and methyl naphthalene; heterocyclic compoundssuch as quinoline and pyridine; ketones such as acetone, methyl ethylketone, and cyclohexanone; esters such as methyl acetate and methylacrylate; amines such as diethylene triamine andN,N-dimethylaminopropylamine; ethers such as diethyl ether, propyleneoxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone(NMP), dimethyl formamide, and dimethyl acetamide; and aprotic polarsolvents such as hexamethyl phospharamide and dimethyl sulfoxide.

Examples of the material of the current collector for a positiveelectrode include metal materials such as aluminum, titanium, tantalum,stainless steel, and nickel, and alloys thereof; and carbon materialssuch as carbon cloth and carbon paper. Preferred is any metal material,especially aluminum or an alloy thereof.

In the case of a metal material, the current collector may be in theform of metal foil, metal cylinder, metal coil, metal plate, metal film,expanded metal, punched metal, metal foam, or the like. In the case of acarbon material, it may be in the form of carbon plate, carbon film,carbon cylinder, or the like. Preferred among these is a metal film. Thefilm may be in the form of mesh, as appropriate. The film may have anythickness, and the thickness is usually 1 μm or greater, preferably 3 μmor greater, more preferably 5 μm or greater, while usually 1 mm orsmaller, preferably 100 μm or smaller, more preferably 50 μm or smaller.The film having a thickness smaller than the above range may haveinsufficient strength as a current collector. In contrast, the filmhaving a thickness greater than the above range may have poorhandleability.

In order to reduce the electric contact resistance between the currentcollector and the positive electrode active material layer, the currentcollector also preferably has a conductive aid applied on the surfacethereof. Examples of the conductive aid include carbon and noble metalssuch as gold, platinum, and silver.

The ratio between the thicknesses of the current collector and thepositive electrode active material layer may be any value, and the ratio{(thickness of positive electrode active material layer on one sideimmediately before injection of electrolyte solution)/(thickness ofcurrent collector)} is preferably 20 or lower, more preferably 15 orlower, most preferably 10 or lower. The ratio is also preferably 0.5 orhigher, more preferably 0.8 or higher, most preferably 1 or higher. Thecurrent collector and the positive electrode active material layershowing a ratio higher than the above range may cause the currentcollector to generate heat due to Joule heating duringhigh-current-density charge and discharge. The current collector and thepositive electrode active material layer showing a ratio lower than theabove range may cause an increased ratio by volume of the currentcollector to the positive electrode active material, reducing thebattery capacity.

The positive electrode may be produced by a usual method. An example ofthe production method is a method in which the positive electrode activematerial is mixed with the aforementioned binder, thickening agent,conductive material, solvent, and other components to form a slurry-likepositive electrode mixture, and then this mixture is applied to acurrent collector, dried, and pressed so as to be densified.

The densification may be achieved using a manual press or a roll press,for example. The density of the positive electrode active material layeris preferably 1.5 g/cm³ or higher, more preferably 2 g/cm³ or higher,still more preferably 2.2 g/cm³ or higher, while preferably 5 g/cm³ orlower, more preferably 4.5 g/cm³ or lower, still more preferably 4 g/cm³or lower. The positive electrode active material layer having a densityhigher than the above range may cause low permeability of theelectrolyte solution toward the vicinity of the interface between thecurrent collector and the active material, and poor charge and dischargecharacteristics particularly at a high current density, failing toprovide high output. The positive electrode active material layer havinga density lower than the above range may cause poor conductivity betweenthe active materials and increase the battery resistance, failing toprovide high output.

In order to improve the stability at high output and high temperature inthe case of using the electrolyte solution of the invention, the area ofthe positive electrode active material layer is preferably largerelative to the outer surface area of an external case of the battery.Specifically, the total area of the positive electrode is preferably 15times or more, more preferably 40 times or more, greater than thesurface area of the external case of the secondary battery. For closed,square-shaped cases, the outer surface area of an external case of thebattery herein means the total area calculated from the dimensions oflength, width, and thickness of the case portion into which apower-generating element is packed except for a protruding portion of aterminal. For closed, cylinder-like cases, the outer surface area of anexternal case of the battery herein means the geometric surface area ofan approximated cylinder of the case portion into which apower-generating element is packed except for a protruding portion of aterminal. The total area of the positive electrode herein means thegeometric surface area of the positive electrode mixture layer oppositeto a mixture layer including the negative electrode active material. Forstructures including a current collector foil and positive electrodemixture layers on both sides of the current collector, the total area ofthe positive electrode is the sum of the areas calculated on therespective sides.

The positive electrode plate may have any thickness. In order to achievea high capacity and high output, the lower limit of the thickness of themixture layer on one side of the current collector excluding thethickness of the base metal foil is preferably 10 μm or greater, morepreferably 20 μm or greater, while preferably 500 μm or smaller, morepreferably 450 μm or smaller.

To a surface of the positive electrode plate may be attached a substancehaving a composition different from the positive electrode plate.Examples of the substance attached to the surface include oxides such asaluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuthoxide; sulfates such as lithium sulfate, sodium sulfate, potassiumsulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate;carbonates such as lithium carbonate, calcium carbonate, and magnesiumcarbonate; and carbon.

<Negative Electrode>

The negative electrode includes a negative electrode active materiallayer containing a negative electrode active material and a currentcollector.

Examples of the negative electrode active material include carbonaceousmaterials that can occlude and release lithium such as pyrolysates oforganic matter under various pyrolysis conditions, artificial graphite,and natural graphite; metal oxide materials that can occlude and releaselithium such as tin oxide and silicon oxide; lithium metals; variouslithium alloys; and lithium-containing metal complex oxide materials.Two or more of these negative electrode active materials may be used inadmixture with each other.

The carbonaceous material that can occlude and release lithium ispreferably artificial graphite produced by high-temperature treatment ofeasily graphitizable pitch from various materials, purified naturalgraphite, or a material obtained by surface treatment on such graphitewith pitch or other organic matter and then carbonization of thesurface-treated graphite. In order to achieve a good balance between theinitial irreversible capacity and the high-current-density charge anddischarge characteristics, the carbonaceous material is more preferablyselected from carbonaceous materials obtained by heat-treating naturalgraphite, artificial graphite, artificial carbonaceous substances, orartificial graphite substances at 400° C. to 3200° C. once or more;carbonaceous materials which allow the negative electrode activematerial layer to include at least two or more carbonaceous mattershaving different crystallinities and/or have an interface between thecarbonaceous matters having the different crystallinities; andcarbonaceous materials which allow the negative electrode activematerial layer to have an interface between at least two or morecarbonaceous matters having different orientations. These carbonaceousmaterials may be used alone or in any combination of two or more at anyratio.

Examples of the carbonaceous materials obtained by heat-treatingartificial carbonaceous substances or artificial graphite substances at400° C. to 3200° C. once or more include coal-based coke,petroleum-based coke, coal-based pitch, petroleum-based pitch, and thoseprepared by oxidizing these pitches; needle coke, pitch coke, and carbonmaterials prepared by partially graphitizing these cokes; pyrolysates oforganic matter such as furnace black, acetylene black, and pitch-basedcarbon fibers; carbonizable organic matter and carbides thereof; andsolutions prepared by dissolving carbonizable organic matter in alow-molecular-weight organic solvent such as benzene, toluene, xylene,quinoline, or n-hexane, and carbides thereof.

The metal material (excluding lithium-titanium complex oxides) to beused as the negative electrode active material may be any compound thatcan occlude and release lithium, and examples thereof include simplelithium, simple metals and alloys that constitute lithium alloys, andoxides, carbides, nitrides, silicides, sulfides, and phosphides thereof.The simple metals and alloys constituting lithium alloys are preferablymaterials containing any of metal and semi-metal elements in Groups 13and 14, more preferably simple metal of aluminum, silicon, and tin(hereinafter, referred to as “specific metal elements”), and alloys andcompounds containing any of these atoms. These materials may be usedalone or in combination of two or more at any ratio.

Examples of the negative electrode active material containing at leastone atom selected from the specific metal elements include simple metalof any one specific metal element, alloys of two or more specific metalelements, alloys of one or two or more specific metal elements and oneor two or more other metal elements, compounds containing one or two ormore specific metal elements, and composite compounds such as oxides,carbides, nitrides, silicides, sulfides, and phosphides of thecompounds. Such a simple metal, alloy, or metal compound used as thenegative electrode active material can lead to a high-capacity battery.

Examples thereof further include compounds in which any of the abovecomposite compounds are complexly bonded with several elements such assimple metals, alloys, and nonmetal elements. Specifically, in the caseof silicon or tin, for example, an alloy of this element and a metalthat does not serve as a negative electrode may be used. In the case oftin, for example, a composite compound including a combination of 5 or 6elements, including tin, a metal (excluding silicon) that serves as anegative electrode, a metal that does not serve as a negative electrode,and a nonmetal element, may be used.

Specific examples thereof include simple Si, SiB₄, SiB₆, Mg₂Si, Ni₂Si,TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂, CrSi₂, Cu₆Si, FeSi₂, MnSi₂, NbSi₂,TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC, Si₃N₄, Si₂N₂O, SiO_(v) (0<v≤2), LiSiO,simple tin, SnSiO₃, LiSnO, Mg₂Sn, and SnO_(w) (0<w≤2).

Examples thereof further include composite materials of Si or Sn used asa first constitutional element, and second and third constitutionalelements. The second constitutional element is at least one selectedfrom cobalt, iron, magnesium, titanium, vanadium, chromium, manganese,nickel, copper, zinc, gallium, and zirconium, for example. The thirdconstitutional element is at least one selected from boron, carbon,aluminum, and phosphorus, for example.

In order to achieve a high battery capacity and excellent batterycharacteristics, the metal material is preferably simple silicon or tin(which may contain trace impurities), SiOv (0<v≤2), SnOw (0<w≤2), aSi—Co—C composite material, a Si—Ni—C composite material, a Sn—Co—Ccomposite material, or a Sn—Ni—C composite material.

The lithium-containing metal complex oxide material to be used as thenegative electrode active material may be any material that can occludeand release lithium. In order to achieve good high-current-densitycharge and discharge characteristics, materials containing titanium andlithium are preferred, lithium-containing metal complex oxide materialscontaining titanium are more preferred, and complex oxides of lithiumand titanium (hereinafter, abbreviated as “lithium titanium complexoxides”) are still more preferred. In other words, use of aspinel-structured lithium titanium complex oxide in the negativeelectrode active material for an electrolyte battery is particularlypreferred because this can markedly reduce the output resistance.

Preferred examples of the lithium titanium complex oxides includecompounds represented by the following formula:

Li_(x)Ti_(y)M_(z)O₄

wherein M is at least one element selected from the group consisting ofNa, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb.

In order to achieve a good balance of the battery performance,particularly preferred among the above compositions are those satisfyingany of the following:

1.2≤x≤1.4, 1.5≤y≤1.7, z=0  (i)

0.9≤x≤1.1, 1.9≤y≤2.1, z=0  (ii)

0.7≤x≤0.9, 2.1≤y≤2.3, z=0.  (iii)

Particularly preferred representative composition of the compound isLi_(4/3)Ti_(5/3)O₄ corresponding to the composition (i), Li₁Ti₂O₄corresponding to the composition (ii), and Li_(4/5)Ti_(11/5)O₄corresponding to the composition (iii). Preferred examples of thestructure satisfying Z≠0 include Li_(4/3)Ti_(4/3)Al_(1/3)O₄.

The negative electrode mixture preferably further contains a binder, athickening agent, and a conductive material.

Examples of the binder include the same binders as those mentioned forthe positive electrode. The proportion of the binder is preferably 0.1%by mass or more, more preferably 0.5% by mass or more, particularlypreferably 0.6% by mass or more, while preferably 20% by mass or less,more preferably 15% by mass or less, still more preferably 10% by massor less, particularly preferably 8% by mass or less, relative to thenegative electrode active material. The binder at a proportion relativeto the negative electrode active material higher than the above rangemay lead to an increased proportion of the binder which fails tocontribute to the battery capacity, causing a low battery capacity. Thebinder at a proportion lower than the above range may cause loweredstrength of the negative electrode.

In particular, in the case of using a rubbery polymer typified by SBR asa main component, the proportion of the binder is usually 0.1% by massor more, preferably 0.5% by mass or more, more preferably 0.6% by massor more, while usually 5% by mass or less, preferably 3% by mass orless, more preferably 2% by mass or less, relative to the negativeelectrode active material. In the case of using a fluoropolymer typifiedby polyvinylidene fluoride as a main component, the proportion of thebinder is usually 1% by mass or more, preferably 2% by mass or more,more preferably 3% by mass or more, while usually 15% by mass or less,preferably 10% by mass or less, more preferably 8% by mass or less,relative to the negative electrode active material.

Examples of the thickening agent include the same thickening agents asthose mentioned for the positive electrode. The proportion of thethickening agent is usually 0.1% by mass or higher, preferably 0.5% bymass or higher, still more preferably 0.6% by mass or higher, whileusually 5% by mass or lower, preferably 3% by mass or lower, still morepreferably 2% by mass or lower, relative to the negative electrodeactive material. The thickening agent at a proportion relative to thenegative electrode active material lower than the above range may causesignificantly poor easiness of application. The thickening agent at aproportion higher than the above range may cause a small proportion ofthe negative electrode active material in the negative electrode activematerial layer, resulting in a low capacity of the battery and highresistance between the negative electrode active materials.

Examples of the conductive material of the negative electrode includemetal materials such as copper and nickel; and carbon materials such asgraphite and carbon black.

The solvent for forming slurry may be any solvent that can dissolve ordisperse the negative electrode active material and the binder, as wellas a thickening agent and a conductive material used as appropriate. Thesolvent may be either an aqueous solvent or an organic solvent.

Examples of the aqueous solvent include water and alcohols. Examples ofthe organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethyl acetamide, methyl ethyl ketone, cyclohexanone,methyl acetate, methyl acrylate, diethyl triamine, N,N-dimethylaminopropyl amine, tetrahydrofuran (THF), toluene, acetone, diethylether, dimethyl acetamide, hexamethyl phospharamide, dimethyl sulfoxide,benzene, xylene, quinoline, pyridine, methyl naphthalene, and hexane.

Examples of the material of the current collector for a negativeelectrode include copper, nickel, and stainless steel. In order toeasily process the material into a film and to minimize the cost, copperfoil is preferred.

The current collector usually has a thickness of 1 μm or greater,preferably 5 μm or greater, while usually 100 μm or smaller, preferably50 μm or smaller. Too thick a negative electrode current collector maycause an excessive reduction in capacity of the whole battery, while toothin a current collector may be difficult to handle.

The negative electrode may be produced by a usual method. An example ofthe production method is a method in which the negative electrodematerial is mixed with the aforementioned binder, thickening agent,conductive material, solvent, and other components to form a slurry-likemixture, and then this mixture is applied to a current collector, dried,and pressed so as to be densified. In the case of using an alloyedmaterial, a thin film layer containing the above negative electrodeactive material (negative electrode active material layer) may beproduced by vapor deposition, sputtering, plating, or the like.

The electrode formed from the negative electrode active material mayhave any structure. The negative electrode active material existing onthe current collector preferably has a density of 1 g·cm⁻³ or higher,more preferably 1.2 g·cm⁻³ or higher, particularly preferably 1.3 g·cm⁻³or higher, while preferably 2.2 g·cm⁻³ or lower, more preferably 2.1g·cm⁻³ or lower, still more preferably 2.0 g·cm⁻³ or lower, particularlypreferably 1.9 g·cm⁻³ or lower. The negative electrode active materialexisting on the current collector having a density higher than the aboverange may cause destruction of the negative electrode active materialparticles, resulting in a high initial irreversible capacity and poorhigh-current-density charge and discharge characteristics due toreduction in permeability of the electrolyte solution toward thevicinity of the interface between the current collector and the negativeelectrode active material. The negative electrode active material havinga density below the above range may cause poor conductivity between thenegative electrode active materials, high battery resistance, and a lowcapacity per unit volume.

The thickness of the negative electrode plate is a design matter inaccordance with the positive electrode plate to be used, and may be anyvalue. The thickness of the mixture layer excluding the thickness of thebase metal foil is usually 15 μm or greater, preferably 20 μm orgreater, more preferably 30 μm or greater, while usually 300 μm orsmaller, preferably 280 μm or smaller, more preferably 250 μm orsmaller.

To a surface of the negative electrode plate may be attached a substancehaving a composition different from the negative electrode plate.Examples of the substance attached to the surface include oxides such asaluminum oxide, silicon oxide, titanium oxide, zirconium oxide,magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuthoxide; sulfates such as lithium sulfate, sodium sulfate, potassiumsulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; andcarbonates such as lithium carbonate, calcium carbonate, and magnesiumcarbonate.

<Separator>

The lithium-ion secondary battery of the invention preferably furtherincludes a separator.

The separator may be formed from any known material and may have anyknown shape as long as the resulting separator is stable to theelectrolyte solution and is excellent in a liquid-retaining ability. Theseparator is preferably in the form of a porous sheet or a nonwovenfabric which is formed from a material stable to the electrolytesolution of the invention, such as resin, glass fiber, or inorganicmatter, and which has an excellent liquid-retaining ability.

Examples of the material of a resin or glass-fiber separator includepolyolefins such as polyethylene and polypropylene, aromatic polyamide,polytetrafluoroethylene, polyether sulfone, and glass filters. Thesematerials may be used alone or in any combination of two or more at anyratio, for example, in the form of a polypropylene/polyethylene bilayerfilm or a polypropylene/polyethylene/polypropylene trilayer film. Inorder to achieve good permeability of the electrolyte solution and agood shut-down effect, the separator is preferably a porous sheet or anonwoven fabric formed from a polyolefin such as polyethylene orpolypropylene.

The separator may have any thickness, and the thickness is usually 1 μmor greater, preferably 5 μm or greater, more preferably 8 μm or greater,while usually 50 μm or smaller, preferably 40 μm or smaller, morepreferably 30 μm or smaller. The separator thinner than the above rangemay have poor insulation and mechanical strength. The separator thickerthan the above range may cause not only poor battery performance such aspoor rate characteristics but also a low energy density of the wholeelectrolyte battery.

The separator which is a porous one such as a porous sheet or a nonwovenfabric may have any porosity. The porosity is usually 20% or higher,preferably 35% or higher, more preferably 45% or higher, while usually90% or lower, preferably 85% or lower, more preferably 75% or lower. Theseparator having a porosity lower than the above range tends to havehigh film resistance, causing poor rate characteristics. The separatorhaving a porosity higher than the above range tends to have lowmechanical strength, causing poor insulation.

The separator may also have any average pore size. The average pore sizeis usually 0.5 μm or smaller, preferably 0.2 μm or smaller, whileusually 0.05 μm or larger. The separator having an average pore sizelarger than the above range may easily cause short circuits. Theseparator having an average pore size smaller than the above range mayhave high film resistance, causing poor rate characteristics.

Examples of the inorganic matter include oxides such as alumina andsilicon dioxide, nitrides such as aluminum nitride and silicon nitride,and sulfates such as barium sulfate and calcium sulfate, each in theform of particles or fibers.

The separator is in the form of a thin film such as a nonwoven fabric, awoven fabric, or a microporous film. The thin film favorably has a poresize of 0.01 to 1 μm and a thickness of 5 to 50 μm. Instead of the aboveseparate thin film, the separator may have a structure in which acomposite porous layer containing particles of the above inorganicmatter is disposed on a surface of one or each of the positive andnegative electrodes using a resin binder. For example, alumina particleshaving a 90% particle size of smaller than 1 μm may be applied to therespective surfaces of the positive electrode with fluororesin used as abinder to form a porous layer.

<Battery Design>

The electrode group may be either a laminate structure including theabove positive and negative electrode plates with the above separator inbetween, or a wound structure including the above positive and negativeelectrode plates in spiral with the above separator in between. Theproportion of the volume of the electrode group in the battery internalvolume (hereinafter, referred to as an electrode group proportion) isusually 40% or higher, preferably 50% or higher, while usually 90% orlower, preferably 80% or lower.

The electrode group proportion lower than the above range may cause alow battery capacity. The electrode group proportion higher than theabove range may cause small void space in the battery. Thus, if thebattery temperature rises to high temperature and thereby the componentsswell and the liquid fraction of the electrolyte solution exhibits highvapor pressure to raise the internal pressure, the batterycharacteristics such as charge and discharge repeatability andhigh-temperature storageability may be impaired and a gas-releasingvalve for releasing the internal pressure toward the outside may beactuated.

The current collecting structure may be any structure. In order to moreeffectively improve the high-current-density charge and dischargeperformance by the electrolyte solution of the invention, the currentcollecting structure is preferably a structure which reduces theresistances at wiring portions and jointing portions. Such reduction ininternal resistance can particularly favorably lead to the effectsachieved with the electrolyte solution of the invention.

In an electrode group having the laminate structure, the metal coreportions of the respective electrode layers are preferably bundled andwelded to a terminal. If an electrode has a large area, the internalresistance is high. Thus, multiple terminals may preferably be disposedin the electrode so as to reduce the resistance. In an electrode grouphaving the wound structure, multiple lead structures may be disposed oneach of the positive electrode and the negative electrode and bundled toa terminal. This can reduce the internal resistance.

The external case may be made of any material that is stable to anelectrolyte solution to be used. Specific examples thereof includemetals such as nickel-plated steel plates, stainless steel, aluminum andaluminum alloys, and magnesium alloys, and a layered film (laminatefilm) of resin and aluminum foil. In order to reduce the weight, a metalsuch as aluminum or an aluminum alloy or a laminate film is favorablyused.

An external case made of metal may have a sealed-up structure formed bywelding the metal by laser welding, resistance welding, or ultrasonicwelding, or a caulking structure using the metal with a resin gasket inbetween. An external case made of a laminate film may have a sealed-upstructure formed by hot-melting resin layers. In order to improve thesealability, a resin which is different from the resin of the laminatefilm may be disposed between the resin layers. Especially, in the caseof forming a sealed-up structure by hot-melting the resin layers withcurrent collecting terminals in between, metal and resin are to bebonded. Thus, the resin to be disposed between the resin layers isfavorably a resin having a polar group or a modified resin having apolar group introduced therein.

The lithium-ion secondary battery of the invention may have any shape,such as a cylindrical shape, a square shape, a laminate shape, a coinshape, or a large-size shape. The shapes and the structures of thepositive electrode, the negative electrode, and the separator may bechanged in accordance with the shape of the battery.

A module including the lithium-ion secondary battery of the invention isalso one aspect of the invention.

In a preferred embodiment, the lithium-ion secondary battery includes apositive electrode, a negative electrode, and the aforementionedelectrolyte solution, the positive electrode including a positiveelectrode current collector and a positive electrode active materiallayer containing a positive electrode active material, the positiveelectrode active material containing Mn. The lithium-ion secondarybattery including a positive electrode active material layer thatcontains a positive electrode active material containing Mn can havemuch better high-temperature storage characteristics.

In order to provide a high-power lithium-ion secondary battery having ahigh energy density, preferred as the positive electrode active materialcontaining Mn are LiMn_(1.5)Ni_(0.5)O₄, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂,and LiNi_(0.6)Co_(0.2)Mn_(0.2) O₂.

The amount of the positive electrode active material in the positiveelectrode active material layer is preferably 80% by mass or more, morepreferably 82% by mass or more, particularly preferably 84% by mass ormore. The upper limit of the amount thereof is preferably 99% by mass orless, more preferably 98% by mass or less. Too small an amount of thepositive electrode active material in the positive electrode activematerial layer may lead to an insufficient electric capacity. Incontrast, too large an amount thereof may lead to insufficient strengthof the positive electrode.

The positive electrode active material layer may further contain aconductive material, a thickening agent, and a binder.

The binder may be any material that is safe against a solvent to be usedin production of electrodes and the electrolyte solution. Examplesthereof include polyvinylidene fluoride, polytetrafluoroethylene,polyethylene, polypropylene, SBR (styrene-butadiene rubber), isoprenerubber, butadiene rubber, ethylene-acrylic acid copolymers,ethylene-methacrylic acid copolymers, polyethylene terephthalate,polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, NBR (acrylonitrile-butadiene rubber), fluoroelastomer,ethylene-propylene rubber, styrene-butadiene-styrene block copolymersand hydrogenated products thereof, EPDM (ethylene-propylene-dieneterpolymers), styrene-ethylene-butadiene-ethylene copolymers,styrene-isoprene-styrene block copolymers and hydrogenated productsthereof, syndiotactic-1,2-polybutadiene, polyvinyl acetate,ethylene-vinyl acetate copolymers, propylene-α-olefin copolymers,fluorinated polyvinylidene fluoride, tetrafluoroethylene-ethylenecopolymers, and polymer compositions having ion conductivity of alkalimetal ions (especially, lithium ions). These substances may be usedalone or in any combination of two or more at any ratio.

The amount of the binder, which is expressed as the proportion of thebinder in the positive electrode active material layer, is usually 0.1%by mass or more, preferably 1% by mass or more, more preferably 1.5% bymass or more. The proportion is also usually 80% by mass or less,preferably 60% by mass or less, still more preferably 40% by mass orless, most preferably 10% by mass or less. Too low a proportion of thebinder may fail to sufficiently hold the positive electrode activematerial and cause insufficient mechanical strength of the positiveelectrode, impairing the battery performance such as cyclecharacteristics. In contrast, too high a proportion thereof may causereduction in battery capacity and conductivity.

Examples of the thickening agent include carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol,oxidized starch, monostarch phosphate, casein, and salts thereof. Theseagents may be used alone or in any combination of two or more at anyratio.

The proportion of the thickening agent relative to the active materialis usually 0.1% by mass or higher, preferably 0.2% by mass or higher,more preferably 0.3% by mass or higher, while usually 5% by mass orlower, preferably 3% by mass or lower, more preferably 2% by mass orlower. The thickening agent at a proportion lower than the above rangemay cause significantly poor easiness of application. The thickeningagent at a proportion higher than the above range may cause a lowproportion of the active material in the positive electrode activematerial layer, resulting in a low capacity of the battery and highresistance between the positive electrode active materials.

The conductive material may be any known conductive material. Specificexamples thereof include metal materials such as copper and nickel, andcarbon materials such as graphite, including natural graphite andartificial graphite, carbon black, including acetylene black, andamorphous carbon, including needle coke. These materials may be usedalone or in any combination of two or more at any ratio. The conductivematerial is used in an amount of usually 0.01% by mass or more,preferably 0.1% by mass or more, more preferably 1% by mass or more,while usually 50% by mass or less, preferably 30% by mass or less, morepreferably 15% by mass or less, in the positive electrode activematerial layer. The conductive material in an amount less than the aboverange may cause insufficient conductivity. In contrast, conductivematerial in an amount more than the above range may cause a low batterycapacity.

In order to further improve the high-temperature storagecharacteristics, the positive electrode current collector is preferablyformed from a valve metal or an alloy thereof. Examples of the valvemetal include aluminum, titanium, tantalum, and chromium. The positiveelectrode current collector is more preferably formed from aluminum oran alloy of aluminum.

In order to further improve the high-temperature storage characteristicsof the lithium-ion secondary battery, a portion in contact with theelectrolyte solution among portions electrically coupled with thepositive electrode current collector is also preferably formed from avalve metal or an alloy thereof. In particular, the external case of thebattery and a portion that is electrically coupled with the positiveelectrode current collector and is in contact with the non-aqueouselectrolyte solution among components accommodated in the external caseof the battery, such as leads and a safety valve, are preferably formedfrom a valve metal or an alloy thereof. Stainless steel coated with avalve metal or an alloy thereof may also be used.

The positive electrode may be produced by the aforementioned method. Anexample of the production method is a method in which the positiveelectrode active material is mixed with the aforementioned binder,thickening agent, conductive material, solvent, and other components toform a slurry-like positive electrode mixture, and then this mixture isapplied to a positive electrode current collector, dried, and pressed soas to be densified.

The structure of the negative electrode is as described above.

The invention may also relate to an electric double-layer capacitorincluding a positive electrode, a negative electrode, and theaforementioned electrolyte solution.

At least one selected from the positive electrode and the negativeelectrode is a polarizable electrode in the electric double-layercapacitor of the invention. Examples of the polarizable electrode and anon-polarizable electrode include the following electrodes specificallydisclosed in JP H09-7896 A.

The polarizable electrode mainly containing activated carbon to be usedin the invention preferably contains inactivated carbon having a largespecific surface area and a conductive material, such as carbon black,providing electronic conductivity. The polarizable electrode may beformed by a variety of methods. For example, a polarizable electrodeincluding activated carbon and carbon black can be produced by mixingactivated carbon powder, carbon black, and phenolic resin, press-moldingthe mixture, and then sintering and activating the mixture in an inertgas atmosphere and water vapor atmosphere. Preferably, this polarizableelectrode is bonded to a current collector using a conductive adhesive,for example.

Alternatively, a polarizable electrode can also be formed by kneadingactivated carbon powder, carbon black, and a binder in the presence ofan alcohol, forming the mixture into a sheet, and then drying the sheet.The binder to be used may be polytetrafluoroethylene, for example.Alternatively, a polarizable electrode integrated with a currentcollector can be produced by mixing activated carbon powder, carbonblack, a binder, and a solvent to form slurry, applying this slurry tometal foil of a current collector, and then drying the slurry.

The electric double-layer capacitor may have polarizable electrodesmainly containing activated carbon as the respective electrodes. Still,the electric double-layer capacitor may have a structure in which anon-polarizable electrode is used on one side. Examples of such astructure include a structure in which a positive electrode mainlycontaining an electrode active material such as a metal oxide iscombined with a polarizable negative electrode mainly containingactivated carbon; and a structure in which a negative electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions or a negative electrode of lithium metal or lithium alloyis combined with a polarizable positive electrode mainly containingactivated carbon.

In place of or in combination with activated carbon, any carbonaceousmaterial may be used, such as carbon black, graphite, expanded graphite,porous carbon, carbon nanotube, carbon nanohorn, and Ketjenblack.

The non-polarizable electrode is preferably an electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions, with this carbon material made to occlude lithium ions inadvance. In this case, the electrolyte used is a lithium salt. Theelectric double-layer capacitor having such a structure can achieve amuch higher withstand voltage exceeding 4 V.

The solvent used in preparation of the slurry in production ofelectrodes is preferably one that dissolves a binder. In accordance withthe type of a binder, the solvent is appropriately selected fromN-methylpyrrolidone, dimethyl formamide, toluene, xylene, isophorone,methyl ethyl ketone, ethyl acetate, methyl acetate, dimethyl phthalate,ethanol, methanol, butanol, and water.

Examples of the activated carbon used for the polarizable electrodeinclude phenol resin-type activated carbon, coconut shell-type activatedcarbon, and petroleum coke-type activated carbon. In order to achieve alarge capacity, petroleum coke-type activated carbon or phenolresin-type activated carbon is preferably used. Examples of methods ofactivating the activated carbon include steam activation and molten KOHactivation. In order to achieve a larger capacity, activated carbonprepared by molten KOH activation is preferably used.

Preferred examples of the conductive agent used for the polarizableelectrode include carbon black, Ketjenblack, acetylene black, naturalgraphite, artificial graphite, metal fiber, conductive titanium oxide,and ruthenium oxide. In order to achieve good conductivity (i.e., lowinternal resistance), and because too large an amount thereof may leadto a decreased capacity of the product, the amount of the conductiveagent such as carbon black used for the polarizable electrode ispreferably 1 to 50% by mass in the sum of the amounts of the activatedcarbon and the conductive agent.

In order to provide an electric double-layer capacitor having a largecapacity and low internal resistance, the activated carbon used for thepolarizable electrode preferably has an average particle size of 20 μmor smaller and a specific surface area of 1500 to 3000 m²/g. Preferredexamples of the carbon material for providing an electrode mainlycontaining a carbon material that can reversibly occlude and releaselithium ions include natural graphite, artificial graphite, graphitizedmesocarbon microsphere, graphitized whisker, vapor-grown carbon fiber,sintered furfuryl alcohol resin, and sintered novolak resin.

The current collector may be any chemically and electrochemicallycorrosion-resistant one. Preferred examples of the current collectorused for the polarizable electrode mainly containing activated carboninclude stainless steel, aluminum, titanium, and tantalum. Particularlypreferred materials in terms of the characteristics and cost of theresulting electric double-layer capacitor are stainless steel andaluminum. Preferred examples of the current collector used for theelectrode mainly containing a carbon material that can reversiblyocclude and release lithium ions include stainless steel, copper, andnickel.

Examples of methods of allowing the carbon material that can reversiblyocclude and release lithium ions to occlude lithium ions in advanceinclude: (1) a method of mixing powdery lithium to a carbon materialthat can reversibly occlude and release lithium ions; (2) a method ofplacing lithium foil on an electrode including a carbon material thatcan reversibly occlude and release lithium ions and a binder so as tobring the lithium foil to be in electrical contact with the electrode,immersing this electrode in an electrolyte solution containing a lithiumsalt dissolved therein so as to ionize the lithium, and allowing thecarbon material to take in the lithium ions; and (3) a method of placingan electrode including a carbon material that can reversibly occlude andrelease lithium ions and a binder on the minus side and placing alithium metal on the plus side, immersing the electrodes in anon-aqueous electrolyte solution containing a lithium salt as anelectrolyte, and supplying a current so that the carbon material isallowed to electrochemically take in the ionized lithium.

Examples of known electric double-layer capacitors include woundelectric double-layer capacitors, laminated electric double-layercapacitors, and coin-type electric double-layer capacitors. The electricdouble-layer capacitor of the invention may also be any of these types.

For example, a wound electric double-layer capacitor may be assembled asfollows. A positive electrode and a negative electrode each of whichincludes a laminate (electrode) of a current collector and an electrodelayer are wound with a separator in between to provide a wound element.This wound element is put into a case made of aluminum, for example. Thecase is filled with an electrolyte solution, preferably a non-aqueouselectrolyte solution, and then sealed with a rubber sealant.

A separator formed from a conventionally known material and having aconventionally known structure may be used also in the invention.Examples thereof include polyethylene porous membranes and nonwovenfabric of polypropylene fiber, glass fiber, or cellulose fiber.

In accordance with any known method, the electric double-layer capacitormay be prepared in the form of a laminated electric double-layercapacitor in which sheet-like positive and negative electrodes arestacked with an electrolyte solution and a separator in between or acoin-type electric double-layer capacitor in which positive and negativeelectrodes are fixed in a coin shape by a gasket with an electrolytesolution and a separator in between.

The electrolyte solution of the invention is useful as an electrolytesolution for large-size lithium-ion secondary batteries for hybridvehicles or distributed generation, and for electric double-layercapacitors.

EXAMPLES

The invention is described with reference to examples, but the examplesare not intended to limit the invention.

Synthesis Example 1 (Method of Synthesizing Compound (1a))

A 500-ml autoclave was charged with hydroxyacetone (70 g), pyridine (15ml), and methylene chloride (40 ml). The components were cooled down to−100° C. with liquid nitrogen, and carbonyl fluoride (65 g) wasintroduced thereinto through a cylinder. The temperature was increasedup to room temperature, and the components were heated at a temperatureinside the container of 70° C. for 22 hours, whereby a black liquid wasobtained. The resulting black liquid was mixed with methylene chlorideand water. The mixture was then subjected to liquid separation, wherebythe target product (compound (1a)) was obtained.

Synthesis Example 2 (Method of Synthesizing Compound (1b))

THF (1.7 g) was added to trifluoromethyl ethylene carbonate (100 mg),and the components were stirred at −78° C. Then, potassium t-butoxide(73.3 mg) and iodomethane (0.05 g) were added thereto and the componentswere stirred for 2 hours. The mixture was quenched with ice water. Thereaction solution was roughly purified, and the resulting roughlypurified product was purified by column chromatography. Thereby, 77.5 mgof the target product (compound (1b)) was obtained.

The following compounds (1c) to (1f) can be produced by known methods.

Examples 1 to 23 and Comparative Examples 1 to 7 (Preparation ofElectrolyte Solution)

The components were mixed in accordance with the composition shown inone of Tables 1 to 3. LiPF₆ was added thereto so as to have aconcentration of 1.0 mol/L, whereby a non-aqueous electrolyte solutionwas obtained.

The components shown in Tables 1 to 3 were as follows.

(Production of Aluminum Laminate-Type Lithium-Ion Secondary Battery)(Production of Positive Electrode)

LiNi_(0.5)Mn_(1.5)O₄ serving as a positive electrode active material,acetylene black serving as a conductive material, and a dispersion ofpolyvinylidene fluoride (PVdF) in N-methyl-2-pyrrolidone serving as abinder were mixed to give slurry. Thereby, positive electrode mixtureslurry was prepared. The positive electrode active material, theconductive material, and the binder had a solid content ratio of 93/2/5(by mass %). This slurry was applied to one surface of 15-μm aluminumfoil and dried. The workpiece was then roll-pressed using a press andcut out into a shape including an active material layer having a widthof 50 mm and a length of 30 mm and an uncoated portion having a width of5 mm and a length of 9 mm. Thereby, a positive electrode was produced.

(Production of Negative Electrode)

A carbonaceous material in an amount of 97 parts by mass was mixed with1 part by mass of an aqueous dispersion of sodium carboxymethylcellulose (concentration of sodium carboxymethyl cellulose: 1.5% bymass) and 1.5 parts by mass of an aqueous dispersion ofstyrene-butadiene rubber (concentration of styrene-butadiene rubber: 50%by mass) serving as a thickening agent and a binder. The components weremixed using a disperser to provide slurry. The resulting slurry wasapplied to 10-μm-thick copper foil and dried. The workpiece was rolledusing a press and cut out into a shape including an active materiallayer having a width of 52 mm and a length of 32 mm and an uncoatedportion having a width of 5 mm and a length of 9 mm. Thereby, a negativeelectrode was produced.

(Production of Aluminum Laminate Cell)

The positive electrode was placed to face the negative electrode with a20-μm-thick porous polyethylene film (separator) in between. Thenon-aqueous electrolyte solution obtained above was poured into theworkpiece and made to sufficiently permeate into the components such asthe separator. The workpiece was then sealed, pre-charged, and aged,whereby a lithium-ion secondary battery was produced.

(Measurement of Battery Characteristics)

For the lithium-ion secondary battery, the high-temperature cyclecharacteristics, the amount of gas, and the IV resistance were examinedas follows.

(Measurement of Battery Characteristics) (Cycle Capacity Retention)

The secondary battery produced above in the state of being sandwichedand pressurized between plates was subjected to constantcurrent-constant voltage charge (hereinafter, referred to as CC/CVcharge) (0.1 C cut off) to 4.9 V at a current corresponding to 2 C at60° C. Then, the battery was discharged to 3 V at a constant current of1 C. This process was counted as one cycle. The initial dischargecapacity was determined from the discharge capacity of the third cycle.Here, 1 C means the current value required for discharging the referencecapacity of a battery in an hour. For example, 0.2 C indicates a ⅕current value thereof. The cycle was again repeated, and the dischargecapacity after 300 cycles was defined as the capacity after cycles. Theratio of the discharge capacity after 300 cycles to the initialdischarge capacity was determined, which was regarded as the cyclecapacity retention (%).

(Discharge capacity after 300 cycles)/(initial dischargecapacity)×100=cycle capacity retention (%)

(Amount of Gas)

The volume of the lithium-ion secondary battery produced and the volumeof the lithium-ion secondary battery after 300 cycles were measured. Theamount of gas generated (ml) was calculated by the following formula.

(Volume after 300 cycles)−(Initial volume)=Amount of gas generated (ml)

(Evaluation of IV Resistance)

The battery after the evaluation of initial discharge capacity wascharged at 25° C. and a constant current of 0.2 C up to half the initialdischarge capacity. The battery was left at 25° C. and discharged at 2.0C, and the voltage at the 10th second was measured. The resistance wascalculated from the voltage drop in discharge, which was taken as the IVresistance.

TABLE 1 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 ple 8 ple 9 10 Composition ofelectrolyte Compound (1) 1a 1b 1c 1d 1e 1f 1b 1b 1b 1a solution Type 4040 40 40 40 40 15 55 65 40 Proportion (vol %) Compound (2) 2a 2a 2a 2a2a 2a 2a 2a 2a 2b Type 60 60 60 60 60 60 85 45 35 60 Proportion (vol %)Other components — — — — — — — — — — Type Proportion (vol %) BatteryAfter storage of Cycle 85 88 83 84 85 86 85 84 80 82 characteristicselectrolyte capacity retention solution (%) IV resistance 2.6 3.1 2.92.6 3.3 3.2 3.2 3.2 3.5 2.7 (Ω) Amount of gas 3.4 3.2 3.7 3.6 3.4 3.43.4 3.3 3.3 3.5 (ml)

TABLE 2 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple11 ple 12 ple 13 ple 14 ple 15 ple 16 ple 17 ple 18 ple 19 ple 20Composition of electrolyte Compound (1) 1a 1a 1a 1a 1b 1b 1b 1b 1b 1bsolution Type 40 40 20 40 30 30 30 30 30 15 Proportion (vol %) 1b 20Compound (2) 2c 2d 2a 2a 2a 2b 2c 2d 2d 2c Type 60 60 60 30 60 60 60 6060 60 Proportion (vol %) 2c 30 Other components — — — — FEC TFMEC DFECEC PC EMC Type 10 10 10 10 10 35 Proportion (vol %) Battery Afterstorage of Cycle 79 77 86 81 84 86 85 82 81 84 characteristicselectrolyte capacity retention solution (%) IV resistance 2.5 2.7 3.03.0 3.0 3.2 3.1 3.2 3.0 3.1 (Ω) Amount of gas 3.6 3.7 3.3 3.5 3.6 3.33.0 3.3 3.2 3.5 (ml)

TABLE 3 Com- Com- Com- Com- Com- Com- Com- par- par- par- par- par- par-par- ative ative ative ative ative ative ative Exam- Exam- Exam- Exam-Exam- Exam- Exam- Exam- Exam- Exam- ple 21 ple 22 ple 23 ple 1 ple 2 ple3 ple 4 ple 5 ple 6 ple 7 Composition of electrolyte Compound (1) 1b 1b1f 1a 1b — — — — — solution Type 15 15 0.5 40 40 Proportion (vol %)Compound (2) 2c 2c 2a — — 2c 2a — 2c 2c Type 50 50 70 90 60 70 70Proportion (vol %) Other components DMC EP TFEMC EMC EMC FEC FEC FECTFMEC DFEC Type 35 35 29.5 60 60 10 40 40 30 30 Proportion (vol %) EMC60 Battery After storage of Cycle 85 83 81 58 63 57 66 55 67 61characteristics electrolyte capacity retention solution (%) IVresistance 3.0 3.1 3.5 6.6 6.9 4.1 5.4 5.1 8.1 8.0 (Ω) Amount of gas 3.63.5 3.5 9.1 8.4 5.1 12.1 14.1 8.1 7.7 (ml)

1. An electrolyte solution comprising a solvent, the solvent containing:a compound (1) represented by the following formula (1):

wherein R^(a), R^(b), R^(c), and R^(d) are the same as or different fromeach other, and are each —H, —F, —CH₃, or —CF₃; at least one of R^(a),R^(b), R^(c), or R^(d) is —F or —CF₃; and at least one of R^(a), R^(b),R^(c), or R^(d) is —CH₃; and a compound (2) represented by the followingformula (2):

wherein R^(e) is a C1-C5 linear or branched alkyl or alkoxy groupoptionally containing an ether bond; R^(f) is a C1-C5 linear or branchedalkyl group optionally containing an ether bond; and at least one ofR^(e) or R^(f) contains a fluorine atom.
 2. The electrolyte solutionaccording to claim 1, wherein the compound (1) is contained in an amountof 0.001 to 99.999% by volume relative to the solvent.
 3. Theelectrolyte solution according to claim 1, wherein the compound (2) iscontained in an amount of 0.001 to 99.999% by volume relative to thesolvent.
 4. The electrolyte solution according to claim 1, wherein thecompound (1) includes at least one selected from the group consisting ofcompounds represented by any of the following formulae (1a), (1b), (1c),(1d), (1e), and (1f)


5. The electrolyte solution according to claim 1, further comprising anelectrolyte salt.
 6. An electrochemical device comprising theelectrolyte solution according to claim
 1. 7. A lithium-ion secondarybattery comprising the electrolyte solution according to claim
 1. 8. Amodule comprising the electrochemical device according to claim 6 or thelithium-ion secondary battery according to claim 7.